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Publication #HS1249

Peach Scab1

Daniel Mancero-Castillo, Mercy Olmstead, and Phillip Harmon2

There are many peach diseases that can affect fruit quality and the marketability of the produce. Blemishes to the skin or within the flesh can be a reason to reject an entire fruit load or significantly reduce the purchasing price.

Peach scab is a disease caused by the fungus Cladosporium carpophilum. The pathogen can infect other fruits and nuts within the Prunus species, like almonds, apricots, nectarines, and plums. Peach scab is common during periods of humid weather because rain splashes the conidia (asexual spores) from the fungus between leaves, twigs, and fruit in the tree canopy, which spreads the disease.

Shoot/Leaf Symptoms

Since spores of peach scab overwinter in raised lesions on shoots and bark, scouting for symptoms during the winter pruning process can help to determine disease management options. Infection in young, green shoots commonly begins with small, slightly raised, reddish-grey oval or circular lesions approximately 0.08 in (2 mm) in diameter. As shoots mature, the lesions expand to 0.1–0.3 in. (3–8 mm) and develop dark brown borders (Figure 1).

Figure 1. 

Peach scab lesions on green current-season peach shoots.


Credit:

H. Scherm, University of Georgia


[Click thumbnail to enlarge.]

Leaf infections are generally less noticeable, and lesions appear on the underside of the leaves. Angular or imperfect circular areas are pale green and approximately 0.03 in. (1 mm) in diameter. Formation of conidia and conidiophores (upright structures that hold conidia) give the lesions an olive to dark green color. Longer and narrower lesions may appear on the midrib and petiole of leaves, with many leaves turning yellow by the end of the growing season. These leaf infections are of little concern, unless the infection is so great that the tree prematurely defoliates.

Fruit Symptoms

Peach scab causes sunken lesions on the skin of fruit (Figure 2). When disease pressure is high, small lesions become noticeable on the young, green fruit. As the fruit mature, these small lesions grow and begin to produce conidia and conidiophore. Large, dark lesions can be found on mature fruit (Figure 3). Older lesions are grey to olive in color, circular, and well-defined. At this stage, lesions are approximately 0.7–0.2 in (2–5 mm) in diameter and a yellowish halo may surround the dark lesions in fruit with significant blush. In nectarines, peach scab lesions may appear pale green with a dark center.

Figure 2. 

Peach scab lesions on young fruit, showing sunken, dark green, imperfect circles where spores are located.


Credit:

Phil Brannen, UGA.


[Click thumbnail to enlarge.]

Peach scab lesions should not be confused with raised scabs often caused by shot-hole disease (Wilsonomyces carpophilus; http://www.ipm.ucdavis.edu/PMG/r602100711.html). Purple margins with light tan centers differentiate shot hole lesions from those caused by peach scab.

The corky cell layer beneath peach scab lesions does not expand as the fruit grows. This causes cracks in the skin that can extend into the peach flesh, generating an entry point for secondary pathogens such as fruit rot organisms or fruit flies. Often, peach scab is found around the stem end of the fruit because of poor spray penetration into the canopy (Figures 3 & 4). Peaches are most susceptible during the shuck-split stage of growth, while nectarines are most susceptible 1–2 weeks after petal fall. While the fruit are most susceptible at early developmental stages, disease management is important from fruit set to harvest in order to prevent significant skin damage.

Figure 3. 

Peach scab lesions on ripening fruit. Lesions occur on the top part of the fruit where water from rain or irrigation splashes spores down on the fruit.


Credit:

Phil Brannen, UGA


[Click thumbnail to enlarge.]

Figure 4. 

Peach scab lesions on mature ‘UFSun’ fruit. Notice the highest concentration of lesions is located in the stem end where spray penetration was poor.


Credit:

M. Olmstead


[Click thumbnail to enlarge.]

Disease Cycle

Peach scab can overwinter as mycelia (filamentous part of the fungi) in lesions or as chlamydospores (large, thick-walled structures) on vegetative tissue or in the bark of 1-year-old shoots. Chlamydospores are the main source of inoculum in an orchard. During the spring and summer, conidia are produced when relative humidity is at least 100% for 24 hours and temperatures exceed 60°F (16°C). The conidia (spores) are spread by wind or by rain splash. They can also be spread by irrigation systems such as those used for overhead frost protection during the early spring (Figure 3). Wind dispersal is relatively minor compared to rain/irrigation splash, the major means by which fungal spores are spread.

In the southeastern United States, the highest risk for infection occurs between petal fall and shuck split. (For more information on peach phenological stages, see http://www.clemson.edu/extension/peach/commercial/files/peachgrowthstages.jpg.) Because they lack fuzz, nectarine fruit can be infected earlier than peaches, so monitoring should begin earlier in the fruit development. In some parts of the southeastern United States, late infections are not of concern because of the long incubation period between infection and the appearance of symptoms (40–70 days); however, late infection remains a concern in Florida, where many of the low-chill peach varieties grown have a fruit developmental period of 70–90 days (for more information, see http://edis.ifas.ufl.edu/mg374).

During spring seasons with frequent precipitation, spray intervals should be shortened and fungicides should be rotated to avoid development of fungicide resistance. Current and historical weather data can be found for various statewide sites using the Florida Automated Weather Network (FAWN; http://fawn.ifas.ufl.edu/). A rainy spring season (compared to the long-term average for your location) will most likely prolong the period of fungicide application for peach scab.

Management

Planning during the orchard establishment phase should include proper site selection. Avoid low-lying areas with poor air circulation and soil drainage. Implementation of a monitoring program based on the presence of lesions on the bark (Figure 1) of the previous years’ growth can help to determine relative potential for infection in the current year. Lesion numbers and sizes can be monitored while pruning and fruit thinning. Furthermore, inoculum sources can be reduced by removing wild or neglected stone-fruit trees growing nearby.

To date, there are no varieties that are resistant to peach scab. Cultural controls are limited to ensuring that proper pruning practices keep the tree canopy open in order to facilitate fungicide spray penetration. Fungicide sprays must be applied just before peak infection periods to provide maximum protection on developing fruit. The first infection period occurs at petal fall, followed by additional infection periods at shuck split, shuck off, and cover sprays as fruit are developing (Table 1). Targeted sprays work well. They will be most effective during periods of high conidial production, from shuck split to 8 weeks after petal fall (Table 2). Fungicide sprays act as a preventive technique; they do not eliminate scab inoculum from the field.

Tables

Table 1. 

Key infection periods and suggested control strategies for optimal peach scab management.

Phenological stage

Suggested control strategies

Petal fall

Fungicide with antisporulant activity can reduce overwintering inoculum on twigs. Petal-fall and shuck-split sprays are key management periods to reduce potential fruit infection severity.

Shuck split

Use fungicides with contact and systemic actions during this period, when numbers of conidia are high.

Early cover sprays

Shorten spray intervals during periods with frequent rain to maintain fungicide protection on susceptible fruit.

Cover sprays

6–8 weeks after petal fall, likelihood of infection decreases. Spray intervals may be lengthened depending upon weather.

Table 2. 

Suggested fungicide options organized by efficacy for peach scab management during key phenological stages, their Fungicide Resistance Action Committee codes (FRAC codes), application rates, effectiveness, re-entry intervals (REI) and pre-harvest intervals (PHI) Adapted from the Southeastern Peach, Nectarine, and Plum Management Guide*. Effectiveness is gauged from (+++++) = Excellent to (+) = Poor.

Material

FRAC code (2014)

Rate/acre

Effectiveness

REI/PHI

Remarks

Petal Fall to 1% Shuck Split

sulfur

M2

9–12 lbs.

+++

24 hrs./0 days

Petal-fall scab sprays are sometimes of little value. However, if conditions are particularly favorable for scab development, no strategy can undo infections that develop because of a missed spray.

or

         

chlorothalonil

M5

3–4 pts.

+++++

12 hrs./do not apply after shuck split

Chlorothalonil provides 14–21 days of scab control. Chlorothalonil is not labeled for use after shuck split.

Bravo Weather Stik

       

Chlorothalonil and captan are severe eye irritants. Although the restricted-entry interval expires after 12 hours, for 7 days after use, entry is permitted only when the following safety measures are provided.

or

       

1. At least one container designed specifically for flushing eyes must be available in operating condition at the mandatory WPS-required decontamination site.

2. Workers must be informed, in a manner they can understand:

• that residues in the treated area may be highly irritating to their eyes.

• that they should take precautions, such as refraining from rubbing their eyes to keep the residues out of their eyes.

• that if they do get residues in their eyes, they should immediately flush their eyes using the eyeflush container that is located at the decontamination site or using other readily available clean water.

• how to operate the eyeflush container.

Bravo Ultrex WDG

 

2.8–3.8 lbs.

   

or

       

Equus 720 or

 

3–4 pts.

   

ECHO 720

       

or

       

captan

M4

4–6 lbs.

++++

24 hrs./0 days

Captan 50W or 80WDG

 

2.5–3.75 lbs.

   

Captec 4L

 

2–3 qts.

   
         

Azoxystrobin

11

9.0–15.5 fl. ozs.

++++

4 hrs./0 days

For peaches only, 9.0–15.5 fl. ozs. can be used for scab control. For scab, begin applications at petal fall and continue at 7- to 14-day intervals per label. Do not apply more than two sequential applications of FRAC code 11 fungicides before alternating with a fungicide that is not in Group 11. For optimal resistance management, use Abound only once per year and follow up with chlorothalonil at shuck split.

Abound

       

Shuck Split to 10% Shuck Off

sulfur

M2

9–12 lbs.

++

24 hrs./0 days

 

or

         

captan

M4

4–6 lbs.

++++

24 hrs./0 days

 

Captan 50W or 80WDG

 

2.5–3.75 lbs.

   

Captan 50W rates may be increased to 8 lbs. /acre for larger trees. Do not exceed 64 lbs. Captan/acre/season. Captan is a severe eye irritant. See above special instructions for Captan safety.

Captec 4L

 

2–3 qts.

     

or

         

Azoxystrobin

11

9.0–15.5 fl. ozs.

++++

4 hrs./0 days

For peaches only, 9.0–15.5 fl. ozs. can be used for scab control. For scab, begin applications at petal fall and continue at 7- to 14-day intervals. Do not apply more than two sequential applications of FRAC code 11 fungicides before alternating with a fungicide that is not in Group 11. For optimal resistance management, use Abound only once per year and follow up with chlorothalonil at shuck split.

Abound

       

7 to 10 Days after Shuck Split Spray

sulfur

M2

9–12 lbs.

++

24 hrs./0 days

The addition of thiophanate-methyl (Topsin-M) at 1.25 lbs./acre can enhance scab control. If thiophanate-methyl is used here, it should be used only once and not in other earlier or later sprays because of potential for resistance.

or

         

captan

M4

4–6 lbs.

++++

24 hrs./0 days

Captan is a severe eye irritant. See above special instructions for Captan safety.

Captan 50W or 80WDG

 

2.5–3.75 lbs.

     

Captec 4L

 

2–3 qts.

     

Early Cover Sprays Before Harvest

sulfur

M2

9–12 lbs

++

24 hrs/0 days

 

or

         

captan

M4

4–6 lbs

++++

24 hrs/0 days

Captan products provide enhanced scab and green fruit rot control.

Captan 50W or 80WDG

 

2.5–3.75 lbs

   

Captan is a severe eye irritant. See above special Instructions for Captan safety.

Captec 4L

 

2–3 qts

     

or

         

Azoxystrobin

11

9.0–15.5 fl. ozs.

++++

4 hrs./0 days

For peaches only, 9.0–15.5 fl. ozs. can be used for scab control. For scab, begin applications at petal fall and continue at 7- to 14-day intervals. Do not apply more than two sequential applications of FRAC code 11 fungicides before alternating with a fungicide that is not in Group 11. For optimal resistance management, use Abound only once per year and follow up with chlorothalonil at shuck split.

Abound

       
           

tebuconazole

3

4 oz.

+++++

12 hrs./0 days

On larger trees, the per-acre rate may be increased to 8 oz. of Elite, Orius or Tebuzol.

Elite 45DF

         

Orius 45DF

         

Tebuzol 45DF

         
           

fenbuconazole

3

2 oz.

++

12 hrs/0 days

 

Indar 75 WSP

         
           

difenoconazole

3 + 9

16–20 fl. oz.

+++

12 hrs/0 days

 

plus

         

anilinopyrimidine

         

cyprodinil

         

Inspire Super

         

QoI/SDHI mix

11 + 7

10.5–14.5 oz

++++

12 hrs/0 days

 

pyraclostrobin

         

plus

         

boscalid

         

Pristine 38W

         

QoI/SDHI mix

11 + 7

4–6.7 fl oz

++++

12 hrs/0 days

Under certain conditions, mixtures of Merivon with adjuvants, additives and/or other products may cause crop injury, particularly to fruit within two weeks of harvest. Do not use Merivon with:

• Emulsifiable concentrate (EC) or solvent-based formulation products.

• Crop oil concentrate (COC), methylated seed oil (MSO) adjuvants.

pyraclostrobin

       

plus

       

fluxapyroxad

       

Merivon

       

or

         
           

pyrazole-4-carboxamides

7

14–20 fl oz

++

12 hrs/0 days

 

Fontelis

         

Cover Sprays After Harvest

sulfur

M2

9–12 lbs

++

24 hrs/0 days

The addition of thiophanate-methyl (Topsin-M) at 1.25 lbs/acre can enhance scab control. If thiophanate-methyl is used here, it should be used only once and not in other earlier or later sprays because of potential for resistance.

or

         

captan

M4

4–6 lbs

++++

24 hrs/0 days

Captan is a severe eye irritant. See above special instructions for Captan safety.

Captan 50W or 80WDG

 

2.5–3.75 lbs

     

Captec 4L

 

2–3 qts

     

Footnotes

1.

This document is HS1249, one of a series of the Horticultural Sciences Department, UF/IFAS Extension. Original publication date July 2014. Visit the EDIS website at http://edis.ifas.ufl.edu.

2.

Daniel Mancero-Castillo, graduate student, Horticultural Sciences Department; Mercy Olmstead, assistant professor and Extension specialist, Horticultural Sciences Department; and Phillip Harmon, associate professor and Extension specialist, Plant Pathology Department; UF/IFAS Extension, Gainesville, FL 32611.


The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other UF/IFAS Extension publications, contact your county's UF/IFAS Extension office.

U.S. Department of Agriculture, UF/IFAS Extension Service, University of Florida, IFAS, Florida A & M University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Nick T. Place, dean for UF/IFAS Extension.