Thanks to the excessively wet weather in many growing regions, tree fruit growers have been (and are continuing to) contend with all types of diseases which favor wet conditions. According to the National Oceanic and Atmospheric Association (NOAA), regional precipitation ranks for May 2019 show that every region overall, with the exception of the Southeast, which is in a slightly below average rainfall pattern, had excessive rates of precipitation.

From a broader perspective, while some of the Pacific Northwest had slightly below average rates of precipitation from June 2018 to May 2019, all other regions have seen elevated precipitation totals for this period. The data are based on measurements dating all the way back to 1895.

For many across the nation, it has been a very wet year.

What this means for tree fruit growers equates to ongoing concerns with various diseases, including fungal leaf disease. Cherry leaf spot is a serious concern at this time, according to Michigan State University. In New York, Pennsylvania and North Carolina, a newly emerging disease – Marssonina leaf spot – first seen in the past year or two in apple trees in these regions, remains a concern.

Cherry Leaf Spot

Cherry leaf spot (CLS) is a serious economic threat and causes problems primarily on sour cherries, although sweet cherries as well as plums are susceptible. Seen across the U.S. and Canada, CLS is most prevalent in the Midwest and Northeast. In the Great Lakes region, farmers are all too familiar with the disease. According to Michigan State University, anything less than 50% defoliation of sour cherry trees by early September is considered acceptable control.

Caused by the fungus, Blumeriella jaapii, which overwinters in fallen leaves, CLS requires well-timed wet weather in order to leave its mark. The fruiting bodies, which release spores, are active from petal fall for a six- or eight-week period, during periods of adequate rainfall.

If conditions are right, the released spores then land on new green leaves. They can germinate if the leaf surface is wet, and then will enter the leaves via stomata. Depending on the temperature, the number of wetness hours needed for germination varies. At 60 – 70º F, a mere five to six hours is needed.

Above and below this ideal temperature range, longer periods of prolonged wetness are required. Incubation times also vary, with symptoms being seen as soon as five days after infection under ideal damp conditions and within that temperature range, and up to two weeks if moisture becomes scarce or temperatures are too high or low.

According to information from Michael A. Ellis, Department of Plant Pathology, Ohio State University Extension, “Serious leaf spot damage usually occurs in years with numerous rainy periods throughout late spring and summer, when repeated secondary infection cycles allow the disease to snowball into an epidemic.”

This is because leaves are susceptible throughout the growing season. Several subsequent infections can occur after initial spore release.

The primary symptom is purple spots on the leaves in May and early June – but these spots are only the first problem. Spots will enlarge, and sticky white spores accumulate when conditions are wet. These spores, or conidia, will then release the second wave of infection if the conditions conducive to initial spring infection remain.

Lesions typically grow together (and may dry out), and in sour cherries often form holes. Yellowing of the leaves before they finally fall off is common too. Trees may be entirely defoliated. Defoliated trees cannot optimally photosynthesize, and as a result trees will be weakened and more susceptible to disease and winter injury. Defoliated trees will show reduced bud set for the two next seasons’ crops. Fruit is affected, becoming soft, if significant levels of defoliation occur early in the season.

Fungicides are effective and available, and some biological control agents, such as certain strains of Bacillus subtilis, may also have some positive effect. All commercially grown cherry cultivars are susceptible to CLS.

Marssonina Leaf Blotch in Apples

This fungal disease is caused by Marssonina coronaria and has recently emerged as a disease of concern in apple trees in New York, Pennsylvania and North Carolina. According to Kari Peter, Ph.D., of Penn State University, the fungus overwinters in leaf debris and releases acospores for a period of three or four weeks. Acospores have been found just prior to the bloom stage of bud development.

Conditions will determine whether the infectious agent can thrive or not. Rain is an essential ingredient for spore release. Prolonged leaf wetness, from precipitation or high humidity, will allow any inoculant present to cause infection. Infection occurs most frequently at temperatures between 68 – 77º F.

This disease was first confirmed in New York and Pennsylvania in 2017. The first confirmed instances in North Carolina occurred in September 2018.

Typical symptoms include individual spots of blotch on upper surfaces of mature leaves in summer. These spots can join together and form holes. Leaf yellowing and lower crown defoliation will be seen in August and September. Fruit infection is uncommon but possible. No fruit infection has yet been found in the states affected.

The disease is controlled by sanitation methods and fungicide sprays. Controlling for the much more common apple scab should also control for Marssonina leaf blotch.

Wet seasons are conducive to all kinds of tree fruit diseases. While it remains to be seen what the rest of the growing season will bring, prolonged wet weather is rarely a good thing.