Diseases

Apple Scab

Venturia inaequalis

Overview

  • Apple scab is caused by the fungus Venturia inaequalis, which infects the leaves and fruit of apples. Scab is one of the most important diseases of apples in New England, and if left unmanaged, will cause significant damage.
  • Infections start in the early spring, caused by spores from leaves infected the previous growing season that overwinter on the orchard floor or close to the orchard border. If infections start, they will produce more spores which can rapidly spread the infection during wet weather in spring and early summer.
  • Management should involve both cultural and chemical control, with fungicide sprays guided by weather conditions and fungicide properties, preferably using disease forecast models and reliable weather data for the orchard site.
  • Sanitation targeting apple leaves in the orchard should be done in fall or early spring to decrease scab risk.
  • Resistance to fungicides is common in apple scab. Application strategies to reduce resistance risk, such as mixing different FRAC groups, including multi-site fungicides, and limiting amounts of any one FRAC group per season should be used.
  • Scab-resistant cultivars have been grown commercially on a limited basis and can eliminate the need for scab fungicides, though some fungicide applications will probably be needed for other diseases.

Symptoms

Apple scab can occur on any apple tissue, but is most commonly seen on leaves and fruit. Small, raised, fuzzy, olive-colored spots will first appear on fruit cluster leaves near bloom, or on early vegetative leaves and immature fruit after petal fall. On leaves, infections may be visible on the top or undersurface. These primary lesions expand if untreated, turning yellow and eventually black. With heavy infections, the entire leaf turns yellow and drops. Leaves that are completely covered with scab are said to have “sheet scab”. Secondary lesions, formed by spores produced in the first or primary lesions, are similar to the primary lesions. They develop on vegetative leaves or fruit through the growing season. Scab infections on fruit first appear as gray to black spots that develop cracks as fruit grows. Tissue becomes brown and corky around and in infections. Multiple infections deform the fruit. In storage, fruit infected at the end of the growing season may develop small, dark spots called “pinpoint scab”.

Disease Cycle

A key to scab management is preventing primary infections early in the growing season. If primary scab is controlled, then there is no need to continue scab fungicide applications during the rest of the growing season. Primary control greatly reduces the chance that resistance to fungicides will develop, and reduces the chance of scab in the next season.

V. inaequalis overwinters in apple leaves infected the previous season that fell to the ground. Scab spores may also overwinter in bark cracks and in buds, and cause infection in the spring, but this is rare in New England. 

Warming temperatures in the spring around bud-break stimulate the fungus to make mature ascospores in old, overwintered leaves. The first mature spores are generally available at the same time trees are first producing green tissue, green tip, though the relative amount of mature spores varies. The first type of scab spores, ascospores, can cause primary infections from green tip until all ascospores for the year have matured and been released, usually one to two weeks after petal fall, though again this timing varies depending on periods of dry weather. In orchards where there were very few or no scab infections the previous year, and where sanitation is done, the risk of infection at green tip is relatively low. The most intense spore release and highest scab risk usually occurs when trees are at pink through to petal fall.

Daytime rains release mature ascospores into the air when they are mature. Those spores may land on emerging apple leaves or new fruit causing primary infections. For infection, apple tissue must be wet for a minimum number of hours, depending on temperature, so it is important to measure the length of wetting periods. Near freezing, the fungus needs two days of leaf wetness to infect, while at 61ºF to 75ºF it takes only 9 hours. After infection, it takes from 9 to 17 days from the time of infection for visible symptoms to show, again depending on temperature. See IApple Scab  Infection Periods.

Primary infections produce conidia, spores that cause secondary infections. Several additional secondary infection cycles can occur during a growing season, depending on rain, though apple tissue becomes more resistant to scab as summer progresses. In the fall, leaves drop and a new generation of ascospores develops the next spring.

Chemical Controls

Most commercial apple cultivars are susceptible to apple scab, and commercial management requires fungicide applications at approximately weekly intervals from bud break to two weeks post-bloom. However, spraying according to the calendar rather the the risk of apple scab will use more fungicide sprays than are usually necessary, so chemicals should be applied according to risk forecasts, most easily obtained using decision support systems, such as Ag-Radar or NEWA.

Detailed options for fungicide selection are given in the Apple Spray Table. The following is a general overview of chemical management of scab.

Early season - silver tip through tight-cluster. A dormant to green tip application of a copper fungicide primarily targeting fire blight is recommended, and is a spray that will also give 5 to 7 days of protection against scab. A combination of captan (3 lb. of Captan 50W or equivalent) plus an EBDC fungicide (3 lb of Dithane M45 or equivalent), a so-called “captozeb” mix, is effective in most orchards at this time. In blocks where scab pressure is high or during extended wet weather, applications should include Syllit, Vangard or Scala. Do not apply more than two applications of these materials in a season.

Tight cluster through pink. If scab pressure is low, the captan/EBDC mix is sufficient. Keep in mind, this is the time when primary scab risk is highest so do not take risks. For moderate to high risk situations, combine a multi-site fungicide, captan or EBDC, with a site-specific fungicide:

  • strobilurine / QoI - Flint or Sovran;
  • SDHI - Fontelis or Aprovia;
  • SDHI plus QoI pre-mix - Luna Sensation, Merivon or Luna Tranquility.

Petal fall through first cover. Again, where and when scab pressure is low a captan/EBDC mix is sufficient. In higher risk situations, combine a multi-site fungicide, captan or EBDC, with a site-specific fungicide:

  • DMI fungicide with high efficacy against scab - Inspire Super, Indar, Topguard, or Rally
  • strobilurine / QoI - Flint or Sovran
  • SDHI - Fontelis or Aprovia
  • SDHI plus QoI pre-mix - Luna Sensation, Merivon or Luna Tranquility. 

There are serious problems with resistance to scab fungicides. See Resistance management: apple fungicides for details.

 

Damage from Scab Fungicides

Mixing captan with oil, other pesticides that contain petroleum-based carriers, or with spreader/sticker/penetrant chemicals may cause damage to fruit and foliage. For tank-mixes that contain several pesticides, thinners or nutrients, avoid using captan. Minimize or avoid use of captan from petal fall through first cover to decrease the risk of fruit damage.

Sanitation

Sanitation targets overwintered inoculum, reducing it and subsequent risk of infection and the magnitude of epidemics that may occur. Leaves on the orchard floor are swept and ground up using mowers or flail choppers in the spring before bud break. In addition, as an alternative, 5% urea may be sprayed on trees just before leaf drop in the fall, or to the orchard floor after leaves drop in fall or spring. Both chopping and a urea spray may be used for more effective control.

Resistant Varieties

Apple varieties vary in their susceptibility to apple scab, and some cultivars are resistant to scab. Cultivars such as McIntosh, Cortland, and Empire are susceptible, while Golden Delicious and related cultivars are less susceptible. Honeycrisp is somewhat resistant to apple scab. Several cultivars have single-gene resistance, for example, Liberty, Modi, Topaz, Pristine, and Ariane, though some scab strains that can infect these varieties have developed in specific areas. Using a few fungicide applications to manage other apple diseases reduces the risk that this resistance will be develop.

Biological Controls

There are an increasing number of biopesticide products that have been labelled for controlling apple scab. However, while they may work reasonably well under low inoculum conditions, they are limited in their ability to manage apple scab under high disease pressure.

Apple scab infection periods

Identifying Apple Scab Infection Periods

The key to managing scab is preventing primary infections. By successfully preventing or limiting the development of primary lesions, the threat of continued infection by conidia is reduced. Since scab infections are invisible for at least 9 to 17 days after infection, is important to understand the conditions that cause a scab infection period to know whether preventative action is needed, or post-infection treatment required. 

Use the Scab Infection Table and measure the length of wetting periods to determine infection periods during the primary scab season.

  • When rain begins during the day (between 8:00 a.m. and 7:00 p.m. in New England under Daylight Savings Time), count the hours of leaf wetness from the first hour rain is recorded until the leaves are dry.
  • Scab ascospores are not released when it is dark, so when rain begins at night (between 7:00 p.m. and 8:00 a.m., DST), count the hours of leaf wetness from 8:00 in the morning until the leaves are dry.
  • The average temperature during leaf wetness should be calculated using hourly temperatures.
  • NEWA Infection Hours are the minimum hours of leaf wetness used by NEWA, as modified in 1989 by MacHardy and Gadoury, from the Jones version of Mills original table. They are adjusted to account for night wetting, and are the most conservative intervals.
  • Low is the minimum hours of wetness required for any infection determined by Jones and Sutton (Diseases of Tree Fruits, North Central Reg. Ext. Pub. 45, 1984).
  • Moderate and High indicate the hours of wetness that will generate significant increases in the amount of infection.
  • Additionional days may be required if conditions are unfavorable for lesion development such as a prolonged period above 80 F or very dry weather.

The process of determining the risk of scab infection can be greatly simplified by using a web-based decision support system, such as Ag Radar or NEWA. For details, contact coordinators through the DSS web sites.

Ag Radar

NEWA

Scab Infection Table

Average

TempErature
oF

newa

Infection
HOURS

Low
infection
hours

Moderate

Infection
HOURS

High

Infection
HOURS

Days

to First 

Symptoms

78

10

13

17

26

 

77

 8

11

14

21

 

76

6.5

9

12

19

 

61 - 75

6

9

12

18

9 - 10

60

6.5

9

13

20

11

57 - 59

7

10

14

22

12 - 13

55 - 56

8

11

15

23

13 - 14

54

8.5

11

16

24

14

52 - 53

9

12

17

25

15

51

10

13

18

27

16

50

11

14

19

29

16

49

11.5

14

20

30

17

48

12

15

20

30

17

47

14

15

23

35

17

46

14

16

24

37

17

45

14

17

26

40

17

44

15

19

28

43

17

43

18

21

30

47

17

42

20

23

33

50

17

41

21

26

37

53

?

40

21

29

41

56

?

39

28

33

45

60

?

38

29

37

50

64

?

37

30

41

55

68

?

36

33

48

72

96

?

35

35

48

72

96

?

34

40

48

72

96

?

Fire Blight

Erwinia amylovora

Overview

  • Fire bight is caused by the bacterium Erwinia amylovora. Outbreaks in New England are sporadic, but have become more common in recent years.
  • Infection of blossoms occurs during warm weather in conjunction with wetting events. Bacteria then migrate through the vascular tissue to the growing shoots and rootstocks killing tissue and whole trees.
  • Fire bight management is a combination of tactics applied every year.
  • Sanitation is accomplished by removing blighted shoots and whole trees.
  • Chemical control begins with a copper spray at silver tip to green tip. Monitor weather data and use a forecast model to determine the need for antibiotics and biopesticides at bloom.  Applications of Apogee or Kudos for shoot blight may be made during active shoot growth.

Symptoms

Fire blight symptoms can show on blossoms, fruit, leaves, shoots, branches and limbs, and rootstocks, and generally are readily recognized. 

Blossoms are often the first tissue to show fire blight symptoms. Infected flowers first have a water-soaked appearance that quickly turns black or brown. Bacteria may spread quickly, first wilting the entire blossom cluster which then turns brown or black, then spreading to adjacent leaves and shoots. Temperature drives symptom development. The warmer the temperature, the sooner symptoms appear and the faster infections spread. After wilting, the tissue may show a sticky white to yellowish ooze produced by the bacteria.

One or more weeks after petal fall, shoot blight can develop. This wilting and browning or blackening of young, vegetative shoots is a classic fire blight symptom, with the young shoot tips bending over into a hook, like the curved end of a cane. Again, if the weather is warm and humid, bacterial ooze droplets may form at the base of the shoot. Shoot blight can expand into older wood, causing dark, sunken cankers. Once in the main trunk of smaller young trees, the whole tree may rapidly wilt, turning brown or black, as if scorched. 

Fire blight cankers are a site where E. amylovora overwinter. As weather warms in the spring, the margins around cankers become less defined as bacteria move into surrounding tissue. Nearby shoots can be infected, producing a symptom called canker blight, where shoots have a characteristic yellow-orange color in the wilting tips in the early weeks after petal fall.

Rootstocks may be more susceptible to fire blight than scions. The pathogen may not cause visible damage to the scion before traveling to the rootstock, where senstive rootstocks will rapidly collapse, killing the tree. Alternatively, rootstocks may be more directly infected if root suckers are present and develop blight.

Disease Cycle

E. amylovora infects many plants in addition to apples and pears, including hawthorn, quince, mountain ash and cotoneaster. All of these plants may be a source of inoculum. The bacteria overwinter in bark and wood at the edges of cankers formed in previous growing seasons. With warm weather, around 65 F, the bacteria multiply and come to the canker surface as sticky ooze droplets. From there, they can be carried to other plants by wind-driven rain or insects.

Insects play a critical role in flower infection, when apples are most likely to become infected with fire blight. Insects deposit bacteria on stigmas in flowers, where they can multiply but typically do not cause infection unless washed to the nectary openings at the base of the flower by rain or heavy dew. Blossoms wilt and die within one to two weeks of infection, producing bacterial ooze that can infect new shoots. Insects and rain move bacteria to shoots, and they infect through microscopic wounds caused by  wind-whipping or possibly insect feeding, or large wounds caused by hail. More bacterial ooze on these shoots, inoculum that can cause new infections as long as shoots keep growing. 

E. amylovora can move systemically in plants, without producing symptoms. The bacteria are also present on non-symptomatic plant surfaces. Their growth on and in plants is driven primarily by temperature. Below 60 F there is little bacterial growth, and low populations of bacteria don't present a fire blight risk. However, at warmer temperatures in the range of 70 to 80 F, especially with high humidity, bacterial populations explode, and soon reach levels that can cause blight. When trees are in bloom, the combination of a high population of bacteria and rain or heavy dew generally lead to infection.

Management

While fire blight is a sporadic disease, when it does show up it can be devastating. Fire blight management involves several tactics at different times in the year, and should be done on an annual basis. Here is a list of tactics to use through the year.

Early Season Copper

Growers should use a dormant, or better, a silver tip/bud swell copper spray every year. Basically, copper is toxic to bacteria on the plant surface, and copper residue will reduce the population of E. amylovora in an orchard. Copper residue is largely depleted after two to three weeks, depending on rain, so does not protect flowers. There are many copper formulations. Apply a minimum of 2 lb. of metallic copper per acre.If in doubt about how much metallic copper a product contains, use the high label rate recommended at silver to green tip. Copper may be used with oil (1 qt./100 gal.), which can act as a spreader/sticker for the copper. Because copper sprays are meant to suppress the population of E. amylovora in an entire orchard, spray the whole orchard, not just the most susceptible cultivars or places where fire blight has occurred in the past.

Monitor for Fire Blight Risk at Bloom

The overwhelming majority of fire blight epidemics start at bloom, and shock waves from these primary infections will reverberate in an orchard through the summer and beyond, so it is essential that blossom blight be stopped. To do this, use a fire blight forecasting model.  There are two basic models, MaryBlyt and CougarBlight, which may be accessed and used independently, but which have also been incorporated into decision support systems linked to weather data on the web. Growers will need either an electronic weather station or a subscription to virtual weather to use these systems, but it makes monitoring much easier. Contact NEWA or Ag-Radar for details.

Spray Antibiotics at Bloom is Needed

If the fire blight forecasting system says that fire blight risk is high, then an antibiotic spray should be applied in bloom. 

Powdery Mildew

Podosphaera leucotricha

Overview

  • Powdery mildew is caused by the fungus Podosphaera leucotricha, which infects the terminal leaves and developing buds of new shoots.
  • Infection begins with overwintering fungus in apple terminal buds. In the spring, infected shoots with shriveled leaves emerge from these buds covered with white, powdery spores, which can cause new infections.
  • Cultural control is best achieved by avoiding cultivars that are highly susceptible to powdery mildew, such as Cortland, Idared, Gingergold, and Jonathan. 
  • Chemical control targeting powdery mildew should be used only in blocks with a history of the disease. Use fungicides that also control apple scab, making sprays at weekly intervals from tight cluster to terminal bud set.

Symptoms and Signs

Primary infections show as poorly growing shoots with whitish, shriveled leaves that are covered with powdery spores. Spores from these shoots cause new infections on terminal leaves.  These first appear as pale yellow infected areas that then produce new spores. Infected leaves curl up, and become covered with a grayish to white fungal growth. These new spores cause additional infections on developing fruit. Fruit infections lead to a webbed russeting, making fruit much less valuable in the market.

Disease Cycle

Unlike most fungal diseases, powdery mildew is worse in warm, dry weather. Regions with cool, wet summers rarely have significant apple powdery mildew problems.

Podosphaera leucotricha overwinters in terminal buds of shoots infected the previous year. These  infections become visible around tight cluster. Spores produced on infected shoots cause secondary infections on leaves and buds, and eventually on developing flowers and fruit. Secondary infections generally appear near petal fall. Infected flowers do not develop normally and produce no fruit. If mildew isn't treated and allowed to grow, it will cover shoots and leaves, and grow into terminal buds where it overwinters. Once growth stops in mid-summer, new infections stop. If powdery mildew inoculum is in an orchard when the fruit cuticle is developing, fruit finish can be damaged showing as a webbed russett on fruit by harvest.

Winter temperatures below 10°F kill some of the over-wintering mildew in buds, and temperatures of –10°F will eliminate 95% of over-wintering mildew. Therefore, powdery mildew is often worse following mild winters, particularly when weather between bloom and petal fall is dry and warm. 

Chemical Control

In New England, spray applications specifically targetting powdery mildew usually are not generally necessary, though warm, dry springs can lead to increased infection of susceptible varieties. For those varieties, in blocks with a history of powdery mildew, scab managment fungicides used from pink through second cover should also have activity against powdery mildew. During extended dry, warm weather during this period, specific applications for mildew may be needed, even when scab fungicide sprays are not.

The most common multi-site fungicides, captan and mancozeb, are ineffective against powdery mildew. DMI fungicides vary in effectiveness. Unfortunately, the DMIs most effective against powdery mildew, Rhyme, Rally, Rubigan and Procure, are least effective against scab, and vice-versa. QoI fungicides, Flint, Flint Extra and Sovran (and pre-mixes with Group 11 ingredients) have good efficacy against powdery mildew. SDHI fungicides, Aprovia, Fontelis and Sercadis (and pre-mixes with Group 7 ingredients) are somewhat less effective, but still provide good control.

Low rates of sulfur are effective in low disease pressure environments, such as New England, but the risk of sulfur injury increases as temperatures go over 85°F. In organic production systems, sulfur applied at weekly intervals, and bicarbonate and peroxide-based fungicides applied on 3-5 days intervals are the best options.

Cultivar Resistance

Apple cultivars differ how susceptible they are to powdery mildew. Cortland, Gala, Ginger Gold,  Idared, Jonathan, Mutsu (Crispin), Paulared, and Rome are all highly susceptible, while Empire, and Fuji are much more resistant.  McIntosh and Golden Delicious can develop significant mildew when they are next to highly-susceptible cultivars that have significant infection.

Cedar-Apple Rusts

Gymnosporangium juniperi-virginianae and other species
Written by: 
Heather Faubert

Overview

Cedar-apple rust is a fungal disease that requires both a Rosaceaous host (such as apple) and a cedar host (such as Eastern red cedar). On apples, bright orange-yellow lesions are first visible after bloom. These lesions develop from spores released from galls on cedars earlier in the spring. Some species of cedar-apple rusts also produce lesions on fruit.

Disease cycle

During prolonged wet periods in the spring, typically from late April to late May in southern New England, galls on cedar/junipers secrete orange-brown gelatinous tendrils (known as telial “horns”) which produce spores. Spores blow from galls and those that land on apple leaves can cause infections if leaves remains wet for 4–6 hours.

Lesions on apple leaves become visible soon after bloom, but continue to enlarge and become more striking in July and August. In late summer, spores are produced on the underside of apple leaves and spread the disease back to cedars.

Management

Fungicides are needed to protect leaves and fruit of susceptible varieties during periods of extended wet weather from pink through 2nd cover. At this time there are no effective organic fungicides that adequately manage cedar-apple rust.

Apple cultivars with resistance to cedar-apple rust include: Baldwin, Delicious (red), Empire, Enterprise, Gala Suprene, Jerseymac, Keepsake, Liberty, McIntosh, Milton, Niagara, Paulared, Redfree, Regent, Sansa, Spartan, Sundance, Viking, and Zestar!.

Apple cultivars that are very susceptible to cedar-apple rust include: Ambrosia, Braeburn, Cameo, Chinook, Crimson Crisp, Fuji, Gala, Ginger Gold, Golden Delicious, Jonathan, Lodi, Prima, Rome Beauty, Shizuka, Spigold, Twenty Ounce, Wealthy, Winter Banana, and York Imperial.

 

Sooty Blotch and Flyspeck

Many different fungi

Overview

  • Sooty blotch and flyspeck (SBFS) are, for practical and management purposes, one disease that blemishes the surface of apple fruit, though several fungi may cause the disease.
  • Initial infections start from spores produced primarily from wild plants along orchard borders, particularly trees and shrubs.
  • Disease activity. The disease is most active from one to two weeks after petal fall through to harvest.
  • Cultural controls such as pruning, mowing and removing plants on orchard borders or planting as far as is practical from woodland and other sources of inoculum decrease SBFS risk.
  • Fungicides are the primary control for SBFS. The initial fungicide application each season can be timed using accumulated leaf wetness hours from petal fall. Later fungicide applications should be timed according the amount of rain or the time that has elapsed since the previous application.

 

Symptoms and Signs

Sooty blotch appears as dark, irregularly shaped areas, like charcoal smudges on fruit. Flyspeck develops distinct black, pinhead-sized spots, generally clustered in groups of 10 to 50. These signs are fungal growth on the surface of apples, not technically symptoms. They often appear together on fruit, but one or the other may occur alone. Details concerning the signs, such as speck size or blotch margins may vary significantly, as many different fungi can cause SBFS. Other than causing cosmetic damage, and rarely some dehydration to fruit in storage, SBFS is not technically a disease, as it does no real harm to apples. However, significant blemishing causes fruit to be downgraded.

 

Disease Cycle

The different life cycles for the many fungi that may contribute to the SBFS disease complex are not well understood. Different species of fungi predominate in different apple production regions, but all have life cycles that are similar enough that symptom development can be reasonably well predicted and a single management approach used. Infection by SBFS fungi occurs soon after fruit set, though symptoms may take several weeks to show, depending on weather. Disease development is dependent on high levels of humidity in the tree canopy. Extended wet weather or periods of high humidity enable SBFS fungi to colonize apples and grow, but they grow slowly if at all during dry periods. New infections can occur throughout the summer to harvest. The fungi may remain invisible for several weeks, first appearing in late summer or early fall. Some SBFS fungi apparently have secondary spore production and infection cycles related to rain and high humidity, with higher rates of disease occurring in years with heavy or frequent rain. These fungi appear to overwinter on plants adjacent to apple orchards.

 

Chemical Control

Fungicides applied approximately every two to three weeks, starting with second cover, will generally control SBFS. Accumulated leaf wetness hours from fruit set (170 to 220 h) can be used to more accurately time the first SBFS fungicide application. After that, timing should be based on the amount of rain and the time from the previous fungicide application. The most effective fungicides against SBFS include the strobilurins, Flint, Sovran, Pristine, and thiophanate-methyl, Topsin, T-Methyl. Captan is not as effective, but provides good control, and is a useful multi-site fungicide to mix with the more effective single-site materials for resistance management. Inspire Super and other pre-mixes that contain a QoI (Luna Sensation, Merivon) also provide good control.

Fungicides should be re-applied when they are depleted, either by rain or breakdown over time. Use the following depletion rules, applying whichever comes first.

  • Pristine (14.5 oz/A) – 2.5 inches rain or 21 days
  • Flint (2.5 oz/A), Sovran (6.4 oz/A), or thiophanate methyl (0.8 lb/A or 20 fl. oz/A) PLUS captan ( 2.5 lb/A Captan 80 or equivalent rate for other formulations) – 2.0 inches of rain or 21 days
  • Captan (4 lb/A Captan 80 or equivalent rate for other formulations) – 1.5 inches of rain or 14 days 

  

Alternative Chemicals. Sulfur, liquid lime sulfur and phosphorous acid compounds (for example Prophyte, Phostrol) also suppress SBFS, though less is known about their depletion rates. With liquid lime sulfur there is risk of russeting on fruit and foliar stress.

SBFS blemishes may be removed or significantly reduced using postharvest fruit dip treatments in low-concentration chlorine bleach solutions (500 to 800 ppm chlorine) followed by brushing on a commercial grading line. 

Non-Chemical Controls

Cultural Control

Anything that slows drying in apple tree canopies encourages SBFS development. So larger trees that are poorly pruned develop more disease. Similarly, trees in areas where air circulation is poor develop more disease. The source of many of the SBFS fungi is wild plant hosts in woods or hedgerows adjacent to orchards. Cutting back these border plants, particularly well-known hosts such as wild blackberries, reduces disease pressure. Keep grass in the orchard mowed to reduce humidity in tree canopies.

Resistant Varieties

Apple cultivars vary in the amount of SBFS at harvest, but this is primarily related to harvest date rather than resistance pathogen colonization. Later harvested cultivars have the highest SBFS incidence. Lower SBFS incidence on the earlier maturing cultivars apparently results from disease avoidance, as these apples are exposed to fewer hours of wetting and high relative humidity, environmental factors favorable for growth of SBFS fungi.

 

 

White Rot and Black Rot

Botryosphaeria species
Written by: 
Adapted from Penn State fact sheets

Overview

The white rot fungus, Botryosphaeria dothidea, often referred to as “Bot rot” or Botryosphaeria rot, can be a distinct canker on twigs, limbs, and trunks. The fungus produces two types of fruit rot, but leaf infections do not occur. Drought stress and winter injury have been associated with an increase in infection and canker expansion. This is a relatively weak fungal pathogen and is only problematic when a tree is stressed, such as due to drought, winter injury, insect damage, or fire blight.

The black rot and frogeye leaf spot fungus, Botryosphaeria obtusa, attacks fruit, leaves, and bark of apple trees and other pomaceous plants.

Symptoms of white rot

New infections on twigs and limbs start to become evident by early summer, appearing as small circular spots or blisters. As the lesions expand, the area becomes slightly depressed.

Cankers stop enlarging in late fall and can be indistinguishable from black rot canker, making isolation of the pathogen necessary for correct identification of the causal organism. By spring small, black pycnidia (the spore-containing structures of the fungus), appear on the smooth surface of new cankers. On older cankers, these may be present throughout the year. Cankers exhibit a scaly, papery outer bark that is often orange and that can easily be peeled off of the tree. Tissues beneath the canker surfaces are watery or slimy and brown. Most cankers are not deep, extending at most to the wood.

Fruit rot infection results in two types of symptoms, depending on the developmental stage of the fruit. One type originates from external infections and the other appears to start internally. External rot is first visible as small, slightly sunken, brown spots that may be surrounded by a red halo. As the decayed area expands, the core becomes rotten and eventually the entire fruit. Red-skinned apple varieties may bleach during the decay process and become a light brown. Because of this characteristic, the disease may be referred to as "white rot."

Symptoms of black rot

The first signs of black rot are small, purple spots appearing on the upper surfaces of leaves and enlarging into circles 1∕8 to ¼ inch in diameter. Leaf margins remain purple, while the centers turn brown, tan, or yellowish brown, giving the lesions a "frogeye" appearance. Small, black pycnidia (pimplelike fruiting bodies of the fungus) may appear in lesion centers.

Infected areas of branches and limbs are reddish brown and are sunken slightly below the level of surrounding healthy bark. These cankers may expand each year, a few eventually reaching several feet in length. The margins of older cankers are slightly raised and lobed, and the bark within their centers usually turns light-colored, loosens, and scales off raggedly. This characteristic is not confined to black rot cankers, so it is not a good diagnostic symptom. Pycnidia form on dead wood of the cankered areas.

Fruit rot usually appears at the calyx end of the fruit. It can originate at any wound that penetrates the epidermis, including insect injuries. There is usually one spot per fruit, a characteristic that distinguishes black rot from bitter rot. Initially, the infected area becomes brown and may not change in color as it increases in size, or it may turn black. As the rotted area increases, often a series of concentric bands form, darker bands of mahogany brown to black alternating with brown bands. The flesh of the decayed area remains firm and leathery. Eventually, the apple completely decays, dries, and shrivels into a mummy. Pycnidia containing spores of the black rot fungus appear on the surface of rotted tissue.

Disease Cycle for white rot

White rot overwinters in fruiting bodies on dead, woody tissue. During spring and summer rains, spores ooze from these structures and are splashed to other parts of the tree. Dead wood and fire-blighted twigs and branches are especially susceptible to invasion, but living twigs, branches, and trunks may also be attacked. Fruit infections can occur at any time from the bloom period to harvest. Infections in young apples usually are not evident until the apples are nearly mature. External rot lesions are found most commonly on the sides of fruit exposed to high temperatures. Drought, heat stress, mechanical wounding, and winter injury favor disease development. The fungus grows best under warm conditions, with the optimum temperature for infection about 86°F.

Disease Cycle for black rot

Black rot can infect from petal fall through harvest. The fungus overwinters in fruiting bodies (pycnidia and perithecia) on dead bark, dead twigs, and mummified fruit. It can invade almost any dead, woody tissue and is frequently found in tissue killed by fire blight. Early leaf infections often are visible as a cone-shaped area on the tree, with a dead twig or mummified fruit at the apex.

In the spring, black pycnidia and perithecia release conidia and ascospores, respectively. Conidia may continue to be produced during wet periods throughout the summer and may remain viable for long periods. When wet, the pycnidium produces a gelatinous coil containing thousands of spores. Disseminated by splashing rains, wind, and insects these spores can infect leaves, the calyxes of blossoms, tiny fruit, and wounds in twigs and limbs. Leaf infection develops during petal fall, at which time conidia attach, germinate in a film of moisture within 5 to 6 hours, and penetrate through stomata or wounds. The optimum temperature for infection is about 68°F. Infections of fruit and wood may not become visible for several weeks.

Initial fruit infections occur during the bloom period but are not usually apparent until midsummer as the apple approaches maturity. Throughout the growing season, infections occur through wounds. Harvest injuries may become infected and the fruit may decay during or after storage, especially if the fruit was harvested during a wet period. Dead fruit spurs or twigs, particularly those killed by fire blight, pruning wounds, winter injuries, and sun scald, are commonly invaded by the black rot fungus.

Management for both diseases

Since stress predisposes apple trees to white rot and black rot, take measures to minimize stressors, such as water stress, winter injury, disease, and insect damage.

Management programs based on sanitation to reduce inoculum levels in the orchard are the primary means of control. Prune out cankers, dead branches, twigs, etc. which serve as inoculum sources and dispose of dead wood. This should be an important component of both current-season and long-range management. Prune and remove cankers at least 15 inches below the basal end; properly dispose of prunings by burial or burning.

Remove mummified fruit of black rot if practical. Cortland apples are especially prone to forming black rot mummies.

Captan + Topsin M and fungicides containing a strobilurin (FRAC Group 11 Fungicides) as an active ingredient are effective at managing white rot and black rot on fruit. Merivon provides excellent control of summer diseases such as white rot, black rot, bitter rot, fly speck and sooty blotch. Note: sterol inhibitor fungicides, such as Indar, Rally, Topguard, have no activity against black rot.

 

Bitter Rot

Colletrotrichum gloeosporioides and C. acutatum.

Overview

  • Bitter rot of apple is more common in the warm, humid climate of the southeatern U.S., and occurs sporadically in the Northeast. Extensive damage can develop rapidly in New England orchards during periods of prolonged hot, wet weather if inoculum sources are present.
  • Bitter rot caused by fungi in the same genus, Colletotrichum, with many different species shown to infect apple fruit. 
  • Colletotrichum spp. infect during extended warm rainy periods after fruit set, continuing through the summer. Infection risk increases as fruit matures.
  • Bitter rot is more common on light or bicolored fruit such as Empire, Honeycrisp, Mclntosh, Sunrise, Paulared and Jonagold.

Symptoms

Typically apples first show bitter rot symptoms in July and August, and fruit susceptibility increases as it matures. Humidity, and the presence of sources of inoculumare also factors that determine when the disease first appears. Bitter rot spots usually appear on the side of the apple directly exposed to the sun. Early fruit lesions are brown, slightly sunken spots. If the apple is cut open, the rotted area beneath the lesion is V-shaped in cross-section. Bitter rot lesions expand most rapidly at a temperature of 86oF. As lesions expand they remain light to dark brown and flattened, developing concentric, target-like rings of spores, colored pink, slight orange to light tan. Severely infected fruit become shriveled and persist on the tree as mummies, a source of inoculum.

Disease Cycle

The disease cycle of the bitter rot fungi related to apple fruit infection is only partially understood. Bitter rot fungi overwinter in dead wood or mummified fruit in apple trees, and may live on other plants, including trees surrounding orchards or broad-leaf weeds in orchards. The fungi produce spores in spring and summer, which are released by rain and dispersed throughout trees. The optimum temperature for spore germination is 79 to 80 F, when it takes only 5 hours to infect. Infections commonly start during prolonged warm, wet weather. As soon as infected fruit produce spores, these can cause new infections. Another source of inoculum is shoots killed by fire blight which become colonized by bitter rot fungi in the same year. 

Chemical Control

Fungicide applications targeting bitter rot should start at fruit set if weather and previous disease history favor infection at that time. For the rest of the summer in orchards with a history of the disease, fungicides to manage bitter rot should be applied when prolonged warm, wet weather is predicted, particularly as fruit matures.  Among the most effective fungicides against bitter rot are the EBDCs. Unfortunately they have a 77 day pre-harvest limit. Captan and Ziram are also effective, as are Pristine and Merivon, group 7 + 11 pre-mixed fungicides. For resistance management and improved efficacy, Pristine or Merivon should be tank-mixed with captan at one half the full rate. No more than 4 applications of Pristine or Merivon should be made per year. Note: Topsin M has no effect on bitter rot.

Cultural Control

The most important cultural control is to remove sources of inoculum: dead wood, branches and shoots with cankers and fruit mummies. Following fire blight outbreaks, dead shoots should be removed as soon as practical as these can become infected with bitter rot and become inoculum sources. While yet unproven, a few other cultural methods may help control bitter rot. It may also be useful to avoid or minimize stress, particularly drought and heat stress. Some tree species close to orchards may support Colletotrichum spp., making it worthwhile to establish open buffer spaces between trees and orchard borders. Removing leaves, dead twigs and fruit mummies on the ground may reduce inoculum. Finally, removing broad-leaf weeds in the orchard may remove another source of inoculum.