Crabby Apples and Other Complaints
We waited a long time for summer to arrive this year, so it is unfair that some flowering crabapples are turning yellow and brown and shedding their leaves already. Mountain-ash, serviceberry, and hawthorn are also affected by the same disorder. Here and there a few maples and other species are also dropping random leaves, which are for the most part still green, often with patches of black or brown. The latter situation has a different origin, but both are rooted in the record-wet spring weather of 2019.
A common pathogen called apple scab (Venturia inaequalis) affects apple trees of course, but quite a few other members of the rose family, including flowering crabapples. Venturia inaequalis is a fungus that overwinters in the fallen leaves of previously infected trees; its spores are released from the old leaves to begin a new infection cycle by the impact of spring rains. Obviously more rain means a greater number of spores in the air and a more severe case of the disease.
Symptoms of apple scab are small brown or olive-green spots on leaves as well as fruit. In a drier season there may be little harm done, but in wet years it often results in many leaves being killed. Sometimes they show a bit of orange or yellow before dropping, though dead leaves may also stay on the branches for the whole season. Apple scab seldom kills trees, but it weakens them. In commercial apple orchards it can lead to blemished fruit that are prone to splitting open.
One of the easiest ways to help minimize apple scab is to rake up and destroy fallen leaves each autumn. Fungicides can reduce symptoms if applied in early spring when buds are just opening. One of the better products is potassium bicarbonate, an organic compound. However, if you have a susceptible flowering crab, it will always be an uphill battle, one which gets worse over time. The very best way to deal with this problem is to replace it with a disease-resistant cultivar. Today there are more than 20 gorgeous cold-hardy crabapples resistant to apple scab. A complete list can be found at http://www.hort.cornell.edu/uhi/outreach/recurbtree/pdfs/~recurbtre...
Anthracnose is a general term for a group of related fungi which infect leaves of many herbaceous plants and hardwood trees. The pathogens are host-specific, so walnut anthracnose is caused by a different organism than maple anthracnose, even though the symptoms are similar. Look for brown or black lesions, usually angular, and bounded by leaf veins. As with apple scab, anthracnose is highly weather-dependent, being far more severe in wet years than dry. Another similarity is that the disease overwinters in leaves that were infected the previous year.
It is harder to control anthracnose because spores can overwinter on twig and branch tissue as well. While fungicide applications may help, shade trees are often too large for a homeowner to effectively reach all the foliage, and it is very expensive to have large trees sprayed with a boom truck. Affected leaves should be raked up and destroyed. In addition, take measures to increase air circulation and sunlight penetration around affected trees. It may be necessary to thin out trees planted too closely.
While both these disorders have been around for centuries, more frequent weather extremes in recent years have made them harder to control than ever. Though there are anthracnose-resistant vegetables, to my knowledge there are no resistant trees other than mango and dogwood, so increased planting distance and better sanitation are essential now. But the number one way to prevent crabby crabapples is to plant only disease-resistant varieties that are happy even when the weather is miserable.