Plums

Renae Moran, University of Maine

Plums are a diverse group of fruits that comprise two major types or species, Prunus domestica, the European plum, and Prunus salciana, the Japanese plum. A third type is a hybrid of the Asian species with American and Canadian plums.  The three types vary in their susceptibility to diseases, but generally have the same insect problems.  Black knot, bacterial leaf spot, brown rot and bacterial canker are the most prevalent diseases in the northeastern United States.  Other diseases of plum include plum leaf spot, plum pockets, phytophthora root rot, and several that are causes by viruses.  Plum curculio and apple maggot are prevalent.  Fruit moths, spotted wing drosophila, plant bugs and trunk borers pose problems, but to a lesser extent in managed orchards.

For additional information on plum production, visit the Plum Production Guide.

Insects

Red sphere hung from a tree branch

A red ball "sticky trap" useful for monitoring apple maggot, a pest of plum.

Insect Pests

Plum curculio, a snout weevil, attacks young fruitlets as they begin to grow to size of one-half inch in diameter.  The plum curculio adult lays eggs directly into the fruit during a period of 5 to 7 weeks after bloom.  In the process of egg laying, the curculio creates a ¼-inch crescent shaped scar on the fruit surface. Infestation leads to fruit drop in most cases, so orchards that are not protected by insecticide can lose a considerable number of fruit to this pest.  Protectant sprays begin at or after petal fall and continue into July.

Apple maggot occurs in abundance in the northeast.  Adult flies lay eggs in fruit in summer, so damage occurs because of fruit larvae feeding within ripening fruit.  Traps, red spheres coated with a layer of sticky gel called Tangletrap™, can be used to monitor populations and to measure when thresholds are reached.  Because populations vary from orchard to orchard, traps can be used to determine when insecticides are most effective or not needed.  Populations are most abundant near unsprayed apple trees. 

Spotted wing drosophila, a fruit fly, can attack ripe fruit. Avoid leaving fully ripe fruit on the tree or in the orchard to minimize damage.  Insecticides applied for apple maggot may also protect fruit from spotted wing drosophila. 

Other pests are likely to be present in the orchard, and are generally managed by insecticide sprays applied to control plum curculio and apple maggot. 

 

Diseases

Bacterial Leaf Spot

Bacterial Leaf and Fruit Spot

The foliage and fruit of many plum, peach and apricot varieties is susceptible to infection by bacterial leaf spot, Xanthomonas arboricola, that causes small yellow spots and a shot-hole appearance in foliage.  Fruit infection leads to round, sunken spots that detract from the appearance and marketability of fruit.  These symptoms can be confused with spray injury.  Warm weather and moisture favor its spread, as do nearby susceptible peach and apricot trees.  Other names for this disease are bacterial shot-hole and bacteriosis.

Plum and peach varieties vary in susceptibility. Shiro and Early Golden have a high degree of susceptibility. Obilinya and Vanier have some resistance. In general, European plums are more tolerant than Japanese varieties. The most effective management strategy is to plant resistant varieties of plum and peach since chemical sprays are of limited use in preventing this disease.  Removal of branches and limbs with cankers during dormant pruning can reduce disease pressure. 

In early summer, signs of bacterial leaf spot first appear as watersoaked spots on the leaves. The shape of the spots is more angular than round. They eventually turn yellow and fall out which creates the shot-hole appearance. The shot-hole effect is more pronounced on plum than peach, whereas in peach, the lesions turn brown with extended areas of yellowing.  Fruit infections lead to sunken spots that easily crack with fruit expansion.

Copper sprays are more effective for prevention than control, and are of limited use due to phytotoxicity.  Consequently, planting resistant varieties is the most effective management strategy. 

Black Knot of Plum and Cherry

  • Black knot infections occur in springtime during the active phase of shoot growth when rainfall favors the spread of this fungus. Wild cherry and plum serve as alternate hosts, spreading the disease to cultivated plums and making chemical control more difficult in susceptible varieties.
  • This disease is common in plum orchards. Symptoms appear in new shoots as green swellings of the stem and oddly shaped growths covering the shoot that eventually turn black with warty surface.  In severe cases, the fungus girdles the branch.  Cultivated plums vary in susceptibility with Stanley and Methely being highly susceptible, and Obilinaja fairly resistant.
  • The most important period for black knot sprays is from white bud to shuck split.  Infections require rain and are favored by temperatures above 55 F.

Disease Cycle and Management

Black knot is caused by the fungus Dibotyron morbosum, also known as Apiosporina morbosa.  It overwinters on infected trees, and is common on wild cherry and wild plum, as well as, ornamental stone fruit species.  Dispersal occurs from ascospores released during rain beginning soon after budbreak at the green cluster stage and continue throughout shoot elongation which continues into summer, but most occur just before and during petal fall.  Symptoms become visible in fall.  Initially, infections appear as green swellings on new shoots, but they eventually turn black in their second year.  Elongated swellings that can vary in length from 1 to 12 inches. Because of their green color, it is easy to miss infections in their first year.  The knot continues to develop through the second summer with ascospores maturing the second winter after infection. 

Infections will affect the woody part of the tree, so with severe infestation, there is a yield reduction. Once established in an orchard, it is difficult to control solely by chemical means. Well-time fungicide sprays can prevent infection. However, where wild infected hosts are near the orchard, complete control is not possible in susceptible varieties.  In this case, infections should be removed during dormant pruning by cutting back several inches into healthy tissue.

Activity Rating of Fungicides

  • Copper (Cuprafix, etc.) - slight
  • Captan - good
  • Chlorothalanil (Bravo) - excellent
  • Sulfur (Microthiol Disperss, etc.) – none (ineffective)
  • Thiophanate-methyl (Topsin M) - fair

Nonchemical control is primarily by removing wild plum and cherry, and by removing infections during pruning.  Where this is not possible, select varieties with partial to complete resistance:

Spring Satin plumcot, Obilinaja, Early Italian, Green Gage, Fellenberg, and President.

 

Brown Rot Blossom Blight and Fruit Rot

  • Plum fruit infected with the brown rot fungus
    Strong resistance to brown rot does not exist in commonly grown varieties, but plums are generally not as susceptible as peaches.  However, European plums are more susceptible than Japanese plums. 
  • The brown rot fungus infects flowers when wet weather occurs during bloom and infects fruit when warm, wet weather occurs during the ripening stage. Infected fruit that is not harvested will dry and persist into the next season to continue the spread of infections, and should therefore be removed during dormant pruning.  
  • Fungicides applied to protect blossoms and ripening fruit should be timed according to susceptibility and weather that favors disease.

Disease Cycle and Management

Brown rot, a fungal disease caused by Monilia fruiticola, infects blossoms, shoots and fruit, and causes significant losses for growers when not controlled.  Plums are less susceptible to brown rot than peaches and cherries, but can be severely infected when conditions are ideal for the fungus.  The brown rot fungus overwinters on mummified fruit that remain attached to twigs, infected blossoms that cling to twigs and on infected shoots.  Fungal conidia are dispersed by wind and rain.  The disease is more severe with wet weather. 

Conidia produced on infected blossoms and shoots spread to fruit.  Infected blossoms wilt, turn brown and cling to the shoot.  The infection can move into the twig and cause a canker.  Blossom infections are not common on plum.

Fruit are susceptible to infection.  Immature fruit can be infected when insect control is poor.  Thinned fruit can serve as a source of innoculum, but thinning fruit so that they do not contact each other will reduce disease at harvest.  Signs of infection begin as small, tan colored circular lesions on the fruit surface.  In warm, humid weather, lesions develop conidia, a mass of grayish brown spores over the fruit surface.  During warm, humid conditions, grayish brown spores are produced which can be diagnostic of brown rot.  Infected fruit that remain in the orchard will shriven and cling to the branch, and serve as a source of disease the following spring.  These shriveled “mummies” should be removed during pruning.

The brown rot fungus can be spread by insects and can also spread from alternate hosts such as wild plum and cherry. Improve air circulation with annual pruning and sucker removal.  Thin fruit so that they are not in contact with other fruit.  Harvest fruit before they become over ripe.

Fungicide resistance has occurred.

Activity Rating of Fungicides

  • Copper – not applicable
  • Captan - good, but phytotoxic to some varieties
  • Chlorothalanil (Bravo, Echo) – excellent for blossom blight, not labeled for fruit rot
  • DMI (Indar, Tebuzol, etc.) – excellent, except for Rally
  • Strobilurin (Pristine) - excellent
  • Strobilurin (Flint, Gem) - suppression
  • Sulfur (Microthiol Disperss, etc.)– fair for blossom blight, slight for fruit rot
  • Biofungicides (Double Nickel, Serifel) - suppression
  • Thiophanate-methyl (Topsin M) – not recommended because of resistance

 

Fruit and leaf spotting and russetting

Russetting spotting on fruit and foliage of a gage plum. Russetting on prune-type plums

 

Possible causes are phytoxic fungicides, particularly Captan.  Russetted spots appear in early summer and can be more prevalent on the sun-exposed side of the fruit.  Captan should not be applied from shuck split through early July, and chlorothalonil from shuck split through harvest.   

Dormant through Summer Sprays Including Plant Growth Regulators

This list is not comprehensive. Other products not listed here may also be labeled for use on plum and plum hybrids.

Commonly Used Products Labeled for Use on Plum, Prune, Pluot and Plumcot

Disease or Pest

IRac/

frac

Product Rate / 100 gal Rate / Acre REI hours PHI days Comments
DORMANT TO BUD BURST

Bacterial canker

Bacterial leaf spot

M1 Badge SC   5-14  pts. 24 NA

Injury to fruit and foliage is possible with application of M1 fungicides during or after bloom.

 

M1

Badge X2

OMRI

  3.5-14 lbs. 48 NA Not after bloom.
M1 Cuprafix Ultra 40 Disperss   5 - 8 lbs. 48 NA Not after bloom.
  M1 Kocide 3000   3.5 - 7 lbs. 24 NA Not after bloom. 
    Cueva   0.5 - 2 gal. 4 1 OMRI. 
  M1 Champ WG   8 - 16 lbs 48 NA Not after bloom.
  M2 Microthiol Disperss   10 - 30 lbs. 24 0 OMRI. Incompatible with oil, do not apply oil 2 weeks before or after sulfur.
    Double Nickel LC   0.5 to 6 qts 4 0 OMRI. Us higher rates for moderate to severe disease pressure.
    Double Nickel 55   0.25 - 3 lbs. 4 0 OMRI.
  44 Serifel Biofungicide   4 - 16 ozs 4 0 OMRI. 

Black knot

M1 Cuprafix Ultra 40 Disperss   3 - 3.75 lbs 48 NA Injury to fruit and foliage is possible with application of M1 fungicides during or after bloom.
M1 Badge SC   3-7 pts. 24 NA
M1

Badge X2

OMRI

  1.5- 5 lbs.  48 NA
  M1

Kocide 3000

Kocide 3000-O

  1.75 - 3.5 lbs. 48 NA.

Supression.  Do not apply after bloom.

O formulation is OMRI.

    Cueva   0.5 to 2 gal. 4 1 OMRI.

Spider Mites

Scale insects  

UN Dormant oil 2 gal. 3 - 6 gal. 12 14  
Scale insects 16 Centaur WDG   34.5 oz. 12 14  
               

WHITE BUD TO PETAL FALL

Bacterial Leaf Spot   Cueva   0.5 - 2 gal. 4 1 OMRI.
    Double Nickel   0.5 - 6 qts. 4 0 OMRI.
  44 Serifel   4 - 16 ozs. 4 0 OMRI.
Black knot M5 Bravo Ultrex 82.5 WDG 0.9 - 1.25 lbs. 2.8 - 3.8 lbs.

12* 

NA  

 

*Severe eye irritant. REI is 7 days with no eyewash station. Special worker training required.

Do not apply from after shuck split through harvest.

M5 Bravo Weather Stik 1 - 1.4 pts. 3.1 - 4.1 pts. 12* NA
M5 other Chlorothanlanil formulations   see label 12* NA
1 Topsin M WSB ⅓ to ½ lb. 1.0 - 1.5 lbs. 48 1  
1 Topsin M 4.5 L 6.7 - 10 fl. oz. 20 - 30 fl. oz. 48 1
M1 Cuprafix Ultra 40 Disperss   3 - 3.75 lbs 48 NA Not after bloom. 
M1 Badge SC   3 - 7 pts. 24 NA Not after bloom.
M1 Badge X2   1.5 - 5 lbs. 48  NA OMRI. Not after bloom.
M1

Kocide 3000

Kocide 3000-O

  1.75 - 3.5 lbs. 48 NA

Not after bloom.

O formulation is OMRI.

  Cueva   0.5 - 2 gals. 4 1  
Brown rot blossom blight M4 Captan 80WDG 1.25 lbs. 3.75 lbs. 24 0

Captan and its different formulations may cause injury on plums if used repeatedly in early season sprays.

Causes irreversible eye damage.

Probable carcinogen.

M4

Captec 4L

Captan FL

1 qt. 1 - 3 qts. 24 0
M4

Captan 50W 

2 lbs. 6 lbs. 24 0
M5 Bravo Weather Stik 1 - 1.4 pts. 3.1 - 4.1 pts.  12* NA

*Severe eye irritant. REI is 7 days with no eyewash station. Special worker training required.

Do not apply from after shuck split through harvest.

 

M5 Other chlorothanil formulations see label    
1 Topsin M WSB ⅓ to ½ lb. 1.0 - 1.5 lbs. 48 1  
1 Topsin 4.5FL 6.7 - 10 fl oz. 20 - 30 fl. oz. 48 1  
2

Rovral

Meteor

  1 - 2 pts. 24 PF Not more than 2 applications per season.  Do not apply after petal  fall.
3, 7 Luna Experience   6 - 10 fl. oz. 12 0  
3, 9 Inspire Super   16 - 20 fl. oz. 12 2 Do not use on sweet cherries.
7, 11 Merivon   4.0 - 6.7 fl. oz. 12 0

May cause crop injury when mixed with some adjuvants.

7, 11 Pristine   10.5 - 14.5 oz. 12 0  
7, 11 Luna Sensation   5 - 7.6 fl oz. 12 1  
3 Indar 2F   6 fl. oz. 12 0  
3 Rhyme   7 fl. oz. 12 7  
3 Cevya   3 to 5 fl. oz. 12 0  
7 Miravis   5.1 fl oz 12 0  
7 Fontelis   14 - 20 fl. oz. 12 0  
11 Flint Extra   2.5 - 3.8 fl. oz. 12 1 Suppression of blossom blight.
M1

Champ WG

 

Champ Formula 2

 

3 lbs.

4.2 pts.

48 NA Apply at popcorn. Not after bloom.
M1 Kocide 3000   3.5 - 5 lbs 48 NA

Not after bloom.

 

M1 Kocide 3000-O   3.5 - 5 lbs. 48 NA OMRI. Not after bloom.
M1 Badge SC   3.5 - 5 pts. 48 NA Not after bloom.
M1 Badge X2   3.5 - 5 lbs. 48 NA OMRI. Not after bloom.
M1 Cuprafix Ultra 40 Disperss   3.75 lbs 48 NA Not after bloom.
M2 Microthiol Disperss   10 - 30 lbs. 24 0

OMRI. Incompatible with oil, do not apply oil 2 weeks before or after sulfur.  Phyotoxic to apricots, d'Anjou pears and Cox O.P. apples.

  Double Nickel LC   0.5 - 6 qts. 4 0

OMRI.

  Double Nickel 55   0.25 - 3 lbs. 4 0 OMRI.
  Cueva   0.5 - 2 gals. 4 1 OMRI.
44 Serifel     4 0 OMRI.
  BotryStop   2 - 4 lbs. 4 0 OMRI.
   
SHUCK SPLIT

Oriental fruit moth

Plum curculio

Leaf rollers

3 Asana XL 2.0 - 5.8 fl. oz. 4.8 - 14.5 fl. oz. 12 14  
4A Assail 30SG   5.3 - 8.0 oz. 12 7  
22 Avaunt eVo   6 oz. 12 14  
3 Danitol 2.4 EC   10.7 - 21.3 fl. oz. 24 3  
5 Delegate 25WG   6 - 7 oz. 4 1

Ineffective for plum curculio.

5 Entrust 80W 0.42 - 0.83 oz. 1.25 - 2.5 oz. 4 7

OMRI. Ineffective for plum curculio.

28 Exirel   13.5 - 20.5 fl. oz. 12 3  
1B Imidan 70WP 0.75 - 1.0 lb. 2.1 - 4.25 lb. 7 14

14 day REI and PHI for hand harvest and the general public (PYO)

1 Sevin XLR Plus, 4F   2 - 3 qts. 12 3  
3A *Mustang Maxx   1.28 - 4.0 fl. oz. 12 14

Do not apply as an Ultra Low Volume spray

4A Actara   4.5 to 5.5 oz 12 14 not for Oriental fruit moth or leaf rollers
UN Surround 95WP 50 lb. 25 - 50 lb. 4 0

OMRI. Suppression only

Stop applications of Surround in early July to prevent noticeable residue at harvest.

3A *Tombstone   2.0 - 2.8 fl oz. 12 7

Causes irreversable eye damage

  3, 28 *Besiege   6 to 12 fl oz 24 12  
Mites   see Additional Summer Sprays      
ADDITIONAL SUMMER SPRAYS

Brown rot

Fungal leaf spot

M4

Captec 4L

Captan FL

1 qt. 3 qts. 24 0

Captan and its different formulations may cause injury on plums if used repeatedly in early season sprays.

Causes irreversible eye damage.

 

M4 Captan 80WDG 1.25 lbs. 3.75 lbs. 24 0
M4 Captan 50 WP 2 lbs. 6 lbs. 24 0
1 Topsin M 4.5L 6.7 - 10 fl oz. 20 - 30 fl oz. 48 1  
1 Topsin M WSB 1/3 to 1/2 lb 1 to 1.5 lb 48 1
7 Fontelis   14 - 20 fl oz. 12 0  
7 Miravis 5.1 fl oz   12 0  
3, 9 Inspire Super   16 - 20 fl oz. 12 2  
7, 11 Merivon   4.0 - 6.7 12 0

May cause crop injury when mixed with adjuvants

7, 11 Pristine   10 - 14.5 oz. 12 0  
3 Indar 2F   6 fl. oz. 12 0  
3 Cevya   3 - 5 fl. oz. 12 0 Maximum of 15 oz. per acre per year
7 Miravis   3.4 to 5.1 fl oz 12 0  
M2 Microthiol Disperss   10 - 20 lbs. 24 0

Incompatible with oil, do not apply oil 2 weeks before or after sulfur.

UN

Double Nickel 55

  0.5 - 1.0 lb. 4 0

OMRI. Suppression only

44 Serifel   0.25 - 1.0 lb. 4 0

OMRI. Suppression only

NC BotryStop   4 lb 4 0 OMRI. Do not use a stomatal flooding or penetrant adjuvant

Plum curculio

Oriental fruit moth

Apple maggot

Leaf roller

Spotted wing drosophila 

Japanese beetle

3 Asana XL 2.0 - 5.8 fl oz. 4.8 - 14.5 fl oz. 12 14  
4A Assail 30 SG   5.3 - 8.0 oz. 12 7  
22 Avaunt   5 - 6 oz. 12 14  
3 Danitol 2.4 EC   10.7 - 21.3 fl oz. 24 3  
4A Actara   4.5 to 5.5 oz 12 14 35-day PHI for apple and pear. 
5 Delegate 25 WP   6 - 7 oz. 4 1

Ineffective for plum curculio and Japanese beetle

5 Entrust 80WP 0.42 - 0.83 oz. 1.25 - 2.5 oz. 4 7

OMRI. Ineffctive for plum curculio and Japanese beetle

28 Exirel   13.5 - 20.5 fl. oz. 12 3  
1B Imidan 70WP 0.75 - 1.0 lb. 2.1 - 4.25 lb. 7 14

14 day REI and PHI for hand harvest and the general public (PYO)

1 Sevin XLR Plus, 4F   2-3 qt. 12 3  
UN Surround WP 50 lbs. 25 - 50 lbs. 4 0

OMRI. Suppression only

To prevent noticeable residue at harvest, cease applications in ealry July.

3A *Mustang Maxx   1.28 - 4.0 fl oz. 12 14

Do not apply as an Ultra Low Volume spray

3A *Tombstone   2.0 - 2.8 fl oz. 12 7

Causes irreversible eye damage.

  3, 28 *Besiege   6 to 12 fl oz 24 14 Not labeled for spotted wing drosophila

Japanese Beetles

Borers

UN Aza-Direct   1 - 3.5 pts. 4 0 OMRI
Mites 6 Agri-Mek   2.2 to 4.2 fl oz 12 21

 

 

  25 Banter WDG   0.75 - 1.0 lbs 12 3  
  23 Envidor 2 SC   16 - 18 fl. oz. 12 7

 

 

  21A Portal XLO   2 pints 12 7

 

 

  10B Zeal   2 - 3 oz. 12 7

Primarily an ovicide

* Restricted use material          
Bird Repellent   Avian Control (methyl anthranilate)   1.9 - 3.75 gal. 4 0 Harvesters must wear gloves and long-sleeved shirts.
PLANT GROWTH REGULATORS
Fruit thinning   None are currently labeled for use on plum.
To increase fruit firmness    ProGibb 40% granular   1.4 - 2.9 oz. 4 0

Apply as a single spray 1 to 4 weeks before harvest

Reduced bloom in the following season can occur.

 

ProGibb 4%

liquid

  16 - 32 fl oz. 4 0
To reduce internal browning in Italian prune   ProGibb 40% granular   1.4 - 4.3 oz. 4 0

Apply as a single spray 4 to 5 weeks before harvest.

Reduced bloom in the following season can occur.

 

ProGibb 4%

liquid

  16 - 48 fl oz. 4 0

Preharvest fruit drop

Fruit firmness maintenance

  ReTain   1 pouch 12 7 Apply 1 to 2 weeks before harvest

Bloom Stages