Apple Blotch Disease

Introduction

Apple blotch (also known as Marssonina leaf blotch) is an emerging disease in the Mid-Atlantic United States. It is caused by the fungus Diplocarpon coronariae (syn. Marssonina coronaria). This disease leads to severe defoliation that indirectly affects the apple fruit size, color, yield and twig development. Apple blotch disease is known to cause serious losses in Asia and Europe, especially on apple scab resistant cultivars. In the United States, it was first reported in Wisconsin (Davis 1903) but was never considered a significant disease before 2017. However, excessive rain during the summer of 2017 in the Mid-Atlantic United States favored outbreaks of apple blotch disease and has caused severe defoliation before harvest. Orchards with reduced fungicide spray programs were severely affected.

Symptoms

Apple blotch disease first starts with symptoms of small circular, purple to brown spots on the surface of the leaves (Fig 1a, b), within which small black asexual fruiting bodies called acervuli form. These bodies are visible as tiny black dots and produce asexual spores called conidia (Fig. 2). With time, smaller leaf spots enlarge, get light brown center with purple edge, and eventually develop to larger brown blotches with margins showing fungal mycelium strands (Fig. 1c). These blotches coalesce, cause yellowing or browning of leaves and lead to premature defoliation. Leaf infections provide inoculum for infections of fruit, which appear as black, concave spots with light-brown center (Fig. 1d). Depending on the cultivar, yellowing surrounding the leaf blotches might or might not appear (Fig. 1c). Severe crown defoliation can reduce marketable yield, affect fruit quality and reduce the overall vigor of apple trees unless fungicides are applied.

Disease Cycle

The life cycle of this disease starts in the spring with spores released by rain splash and aerosol from overwintered leaves on the orchard floor. Spore dissemination and infections occur during or after long periods of rainfall at temperatures from 20 – 25°C (68 – 77°F). Once spores reach the leaves, they germinate into an infection hypha and penetrate the cuticle of the leaf. After incubation, leaf spots become visible and acervuli form on the leaf surface. If the weather conditions are conducive, defoliation can start as early as 2 weeks after the first spot symptoms appear. First leaf spots usually appear between June to July in the northern hemisphere.

Control Options

Apple blotch disease is managed by orchard sanitation, cultural practices such as pruning, and timely fungicide applications.

Cultural Control

The incidence of apple blotch disease can be reduced by orchard sanitation and cultural practices. Shredding the fallen leaves on the orchard floor which serve as sources of inoculum, by flail mower or raking and burning the leaves can reduce the spore dose next spring. Dormant pruning should modify the tree crown to allow good air circulation inside the canopy and rapid foliar drying after rainfall or dew. This diminishes conducive microclimate conditions inside and between the trees for infections and allows better fungicide coverage increasing their efficacy. Application of 40 lbs of urea/A in 100 gals of water onto the overwinter leaves and raking the leaves from under the trees for later shredding by flail mower can reduce the inoculum level of this disease. Avoid planting highly susceptible cultivars such as ‘Rome’, ‘Mutsu’, ‘Royal Gala’, and ‘Golden Delicious’.

Organic Control

Application of dolomitic lime (2.5 tons per acre) just before leaf drop in fall or early in the winter can help apple leaf litter breakdown and thus aids to reduction of the spore inoculum dose in the overwintering leaves (this rate can also be used in apple orchards with synthetic fungicide programs). During the season, 10 – 12 spray applications per year of Funguran (copper hydroxide), sulfur, or lime sulfur can provide good control of leaf infections.

Chemical Control

Fungicides applied during the late spring and summer for control of other diseases like Sooty Blotch and Flyspeck and apple fruit rots are usually effective in controlling this disease. The first fungicide labeled for apple blotch control in the United States is Cevya (EPA Reg. No. 7969-407) with the active ingredient mefentrifluconazole (FRAC Group 3). However, if the fungicide applications are not delivered in intervals dictated by the frequency and amount of rain events, their efficacy can fail (fungicides are ineffective after 2 inches of rain occurring in a single or several wetting events). Whenever possible, fungicides should be integrated with the aforementioned cultural control practices. Fungicide applications are critical to protect leaves from this disease and prevent production of spores on them. Rotation of the most effective fungicides with different modes of action in FRAC Groups 3, 7, or 9 is recommended to prevent fungicide resistance that has been reported in other countries.

Germplasm Resistance

Several resistant cultivars have been bred in China, such as Yepingguo 9 and Dianchihaitang. Currently, in the U.S.A., there are no breeding programs that are developing cultivars resistant to apple blotch disease. However, cultivars like ‘Granny Smith’, ‘Pink Lady’, and ‘Gibson’s Golden’ are moderately resistant. If resistant cultivars become available in the future, they would be one of the key strategies for the integrated management of apple blotch disease.

Disease Infection Prediction

The infections by D. coronariae can be predicted by using a RIMpro Marssonina coronaria model (RIMpro B.V., Amsterdam, Netherlands), available by subscription at: https://www.rimpro.eu/. The model can use the U.S. National Weather Service weather forecast and can be connected to the local weather stations in the Network for Environment and Weather Applications (NEWA) available at: https://newa.cornell.edu/, to allow accurate model infection predictions and thus guide timely fungicide applications.

References