Apple scab

Picture of Apple Scab

The cool wet weather in southern Minnesota during April, May and early June this year has contributed to abundant apple scab infections, and numerous calls from concerned landowners. Susceptible tree crowns are thin with withering leaves rapidly dropping to the ground. Unlike the fungi involved in most other leaf spot diseases, the apply scab fungus remains active through out the season. As long as there is sufficient moisture, new infections can multiply exponentially, creating an epidemic among susceptible hosts. Affected tree species include apple, crabapple, hawthorn, mountain ash and pyracantha.

Life Cycle

The apple scab fungus, Venturia inaequalis overwinters in leaves that have fallen to the ground. Ascospores develop during the winter and mature during the spring. As batches of spores mature, they are forcibly released into the air during rain events, with peak spore release occurring during bloom. Wind currents then carry the spores to newly expanding buds, where a film of water is necessary for successful spore germination and infection. Infection occurs most rapidly when fruit and leaves remain wet for a minimum of 9 hours and temperatures are between 55° and 75°F. If spring weather is dry from the green tip stage through fruit set, apple scab will not likely be a problem.

The buds become susceptible as they swell and become green (before the leaves appear), a stage called the green tip stage (see photo). The young leaves are the most susceptible within one to five days of unfolding (see photo). They become resistant once they are fully expanded and remain so until just before fall leaf drop. Primary (ascospore) infections are usually more common on the lower leaf surfaces because they are the first surface to be exposed. As the fungus infects the newly expanding leaves, brown to olive-green lesions with fringed margins begin to appear (see photo). As they age, these lesions begin to darken and produce millions of secondary spores, called conidia. As the conidia rain down on the leaves below, secondary lesions appear on upper leaf surfaces. As the lesions multiply, infected leaves turn yellow, curl and drop to the ground (see photo). Secondary lesions can be so numerous they take on a velvety sheet-like appearance on the leaves. During years with especially wet springs, entire trees can be defoliated by the end of June (see the life cycle below).

Picture of Apple Scab

Flower buds become susceptible just as they expand and show pink. Early infections can cause the blossoms to drop or young fruit to abort. Although the fruit becomes less susceptible to infection over time, prolonged wet and foggy weather can contribute to new infections any time during the season. Primary (ascospore) lesions on the fruit, commonly on the blossom end, are similar to those on the leaves, except that the margins are more distinct and the lesions become hard and scabby with time (see photo). As they expand, the fruit can become distorted and cracked around these infection sites. Badly infected fruit can become deformed and fall before reaching maturity. Secondary infections generally stay small but can be so numerous that they cover the fruit with tiny dots that may not become evident until after fruit harvest and storage. Disease Management

Picture of Apple Scab

For residential landowners it is important to note that apple scab does not usually affect the long-term health of the trees. But the disease can cause severe defoliation and loss of a fruit crop. It also can lead to reduced growth, susceptibility to secondary pest problems and sensitivity to winter injury. However, the trees are not usually at risk of dying and recover during years with less rain.

The first strategy in the management of apple scab is planting disease resistant varieties (see Tables 1 and 2). There are a number of them in a range flower colors, so homeowners can pick and choose those that appeal to them. Be sure to check for susceptibility to fire blight as well as Japanese beetle before making a final decision. Most reputable nurserymen can tell you which varieties they carry that are going to meet those criteria.

If you inherent a susceptible tree you don?t wish to replace, the next best strategy is sanitation. Because the fungus overwinters in the fallen leaves, homeowners can rake and destroy fallen leaves and thus limit the amount of inoculum (infective material) available for primary infections the following year. However it is important to rake fallen leaves before they have the chance to dry and break up into bits too small to gather up. Even with raking, new spores may blow into your yard from the surrounding area. The number of spores and thus new primary infections is likely to be low - unless there are other fruit trees in close proximity that can provide a ready source of inoculum. Even then, spraying is not usually warranted in most yard situations.

Prune or thin fruit trees and remove suckers during the winter to maintain canopies in an open condition. This will improve air circulation and shorten the time necessary to dry leaf surfaces. It will also improve the penetration of spray materials, if fungicide applications become necessary. Be sure to set irrigation systems such that they do not wet leaf surfaces during the peak infection period.

If fruit production is the concern, then management strategies become more important and more complex. Apple scab is the single most damaging fungal disease in the apple industry. As a result, the serious grower will likely have to supplement cultural practices with a thoughtful fungicide program. Fortunately there has been a lot of work to help predict the likelihood of serious infection and the most effective time to spray.

Picture of Apple Scab

In general, treatments begin just as the flower buds begin to show pink. Treatments then need to be continued on a 7-10 day cycle until the weather turns warm and dry. The main objective is to reduce the number of primary infections so secondary infections do not become serious. There are a number of fungicides labeled for apple scab, several of which are available to homeowners as well as growers (see Table 3). To double check those listed for use in Minnesota, go to www.kellysolutions.com, a commercial site sponsored by the MN Dept. of Agriculture, that lists all products labeled for use in Minnesota. Most fungicides act as protectants and as such, need to be applied to leaf surfaces before spores are deposited. In addition to the list of fungicides provided, there are also a number of general yard and garden products that contain one or more of these fungicides. However, if using a general product, check to see if it also contains an insecticide, as these can be toxic to honeybees

Monitoring

Spring inoculum levels can be assessed the previous fall. Randomly select shoots through out the tree canopy, including sucker sprouts and top growth, and count the number of scabbed leaves (not the number of lesions). To get an accurate count, a total of 600 shoots should be surveyed. If the number of infected leaves is less than 50, you can assume spring inoculum levels will be low. If the number is 50-100, a sanitation program will be needed, but may be sufficient without chemical treatments. If the number of infected leaves is greater than 100, a well-timed treatment program will be necessary the following season to ensure successful fruit production. Because the timing of fungicide applications is so critical, effective weather monitoring is key to a successful business operation. So commercial growers are advised to invest in weather monitoring equipment. Temperature, relative humidity and leaf wetness are all important factors in determining the risk of infection. For a full discussion of monitoring methods and treatment regimes, see www.gov.on.ca/OMAFRA/english/crops/facts/apscab.htm.


Table 1. Susceptibility of Apple Varieties to Apple Scab (UC Davis, ?01)

Susceptible

Highly Resistant

Bellflower

Monroe

Easy-Crisp

Mac-free

Blushing gold

Mutsu

Enterprise

Prima

Fujii

Paula Red

Florina

Priscilla

Gala

Red Delicious

Freedom

Pristine

Golden Delicious

Rome Beauty

Gold Rush

Redfree

Granny Smith

Stayman Winesap

Jon Grimes

Prize

Gravenstein

Winesap

Jonafree

Spigold

Grimes

Yellow Newtown

Liberty

Williams Pride

Ida Red

York Imperiol

 

 

Jonathon

 

 

 




Table 2. Susceptibility of Crabapple Varieties to Apple Scab (Purdue, ?02)

Highly Susceptible

Moderately Resistant

Highly Resistant

Adams

Canary

Ann E.

Brandywine

Candymint Sargent

Basakatong

Candied Apple

Centurion

Bob White

Indian Magic

David

Jack

Indian Summer

Donald Wyman

Japanese Flowering

Profusion

Doubloons

Lousia

Robinson

Harvest Gold

Ormiston Roy

Snowdrift

Jewelberry

Prairie Maid

Velvet Pillar

Liset

Prairifire

White Candle

Madonna

Redbud

White Cascade

Mary Potter

Red Jewel

 

Molten Lava

Sargent

 

Selkirk

Sinai Fire

 

Sentinel

Sugar Tyme

 

Silver Drift

Tea

 

 

White Angel




Table 3. Fungicides currently labeled for apple scab (taken in part from U of CT, 03)

Chemical Name

Trade Names

Captan

Captan

chlorothalonil**

Daconil 2787, Bravo, Fung-On, Eco 75WG

thiophanate-methyl**

Cleary's 3336, Halt, Domain, Topsin-M

Ferbam

Ferbam

mancozeb

Dithane, Fore, Manzate, Penncozeb

Propiconazole

Banner MAXX

harpin protein

Messenger

copper, copper sulfate, copper hydroxide

Kocide, Phyton 27, Cuprofix, Disperss Bordeaux

 

**Ornamental Trees only, see the websites listed above for additional products available for use on edible fruit trees.