Frogeye leaf spot on apple and crabapple is showing symptoms now in the landscape: circular spots on young leaves with indefinite purple margins. Later they will turn brown and if they regrow, they become lobed giving the frogeye pattern. Sometimes susceptible crabapples can be defoliated prematurely in wet seasons. Under most conditions, control is not necessary, but the same fungicides applied for scab will control frogeye.
Host plants:
Frogeye (Sphaeropsis) leaf spot occurs on apple and crabapple. There are frogeye leaf spots on leaves of other trees and shrubs. The appearance of the leaf spot is similar, but fungi other than Sphaeropsis cause these infections.
Description:
The concentric pattern of light brown to tan center portions of the leaf spot ringed by darker purple margins is the origin the name “frogeye leaf spot.” Young leaves exhibit circular spots with indistinct purple edges about 2 weeks after petal fall. Many of the spots develop no further and a well-defined circular brown spot is present by summer. However, in other spots, there is secondary enlargement during the summer and the brown spots develop irregular discolored lobes. Tiny black fruiting structures often develop in the center of the leaf spots. Extensive spotting of leaves initially causes chlorosis followed by early leaf loss.
Disease cycle:
Fruiting structures develop in black rot cankers on apple and crabapple branches and twigs in the spring and they release spores via wind and rain splash. If spores land on wet unfolding leaves, they germinate and penetrate the leaves through stomata. Later that season fruiting structures develop in the spots and these can serve as an inoculum source for black rot canker infections on nearby apple and crabapple branches with wounds or cracks in the bark.
Management strategies:
Usually, frogeye leaf spot alone does not threaten the health of apple and crabapple trees. However, it can be an important source of spores for black rot canker infections that cause extensive branch dieback. Remove and dispose of cankered branches when conditions are dry to reduce inoculum available to initiate frogeye leaf spot and well as cankers. Use fungicide treatments to prevent infections of fruit in orchard situations if sanitation does not sufficiently suppress fruit spot infections.
Information from Bob Mulrooney, Extension Plant Pathologist, UD and Daniel H. Gillman, Plant Pathologist, UMass Extension Landscape, Nursery & Urban Forestry Program
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