2026 Season Update from the UVM Diagnostic Clinic – May

2026 Season Update from the UVM Diagnostic Clinic – May 21, 2026

 

It has been a cold rainy start to spring, and although I have not seen a lot of problems yet, I can probably predict what is coming. I suspect we will see a fair amount of anthracnose in maple, ash and sycamore, much like last year. This fungal disease tends to cause browning along the leaf veins and when severe it can result in defoliation. It is more of an aesthetic problem and rarely causes any harm to the tree.  For pictures and more information.

We have had several significant apple scab infection periods with the recent rains. This will result in lots of foliar spotting and early defoliation of crabs and apples. To control this fungal disease requires several fungicide sprays through the end of June, but in the landscape the best way to manage the disease is by opening up the tree to air and light so leaves dry quicker. Choosing resistant cultivars is the best way to avoid the disease. https://extension.umn.edu/plant-diseases/apple-scab. The long cool and rainy period will also keep cedar apple rust galls active for longer periods, releasing spores that travel up to 5 miles to an apple host. This disease also contributes to the early defoliation of crabs and apples.  Clipping and destroying the galls on the cedar host will help reduce inoculum (spores) and protection of commercial apples is accomplished through fungicides. https://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/insects-pests-and-problems/diseases/rusts/cedar-apple-rust. With the upcoming 80F temperature predictions of next week, also comes a higher risk for fireblight in apples, crabs, hawthorn and mountain ash.  Many parts of the state will be in full bloom and with conducive temperatures and moisture, bees can move the disease from blossom to blossom. If you do see ‘strikes’ (dead new growth that looks like it has been burned) prune them out with sterilized pruning shears on a dry day and destroy.

I also suspect we will see the lilac leaf blight again for the third year. The fungus, Pseudocercospora, overwinters on fallen leaves and the spores are splashed or carried by wind to new foliage. Raking and destroying leaves, opening up the bush to air and light will all help minimize the disease. So far, it seems the lilacs have bounced back from this, but this stress may weaken them over time. Protectant fungicides may help if sprayed to protect the new foliage. https://www.uvm.edu/extension/news/lilac-leaf-blight.

Peach leaf curl may be a problem on peaches and nectarines. The same genus of fungus can also cause oak leaf blister (raised blister-like lesions on the leaves) and plum pockets (bladder-like growths of the fruit). Peach leaf curl causes curled and distorted and often pinkish leaves. Once you see any of the 3 diseases, it is too late for control, and none will harm the trees. For pictures and more info on the Taphrina diseases:

• Peach leaf curl https://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/insects-pests-and-problems/diseases/fungal-spots/peach-leaf-curl.

• Plum pockets https://extension.umn.edu/plant-diseases/plum-pockets.

• Oak leaf blister https://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/insects-pests-and-problems/diseases/fungal-spots/oak-leaf-blister.

Wet weather as spruce and firs are budding out and expanding increases the chances of infection of the new needles to Rhizosphaera needlecast. The black fruiting bodies in rows can be seen on the undersides of last year’s browned needles. This common disease causes needle drop and death of branches from the bottom up. Young specimen trees can be protected with fungicides, but it is pretty impossible/expensive to treat older trees. Planting more resistant Norway and White spruce as opposed to very susceptible Blue Spruce is a good idea. https://extension.umn.edu/plant-diseases/rhizosphaera-needle-cast.

It still remains to be seen what the impacts are this spring from last season’s ‘flash drought.’ I have heard/seen some trees that have totally failed to leaf out, and some (evergreens) that looked good in the fall but have a lot of browning in the spring. I suspect some form of winter injury/desiccation injury. Typically, this would not kill a tree but even though we were basically out of the drought by fall, I wonder if a lot of trees went into the winter from the earlier stress and the winter was kind of the final straw.

I have also received a few pictures (mulberry, elderberry, magnolia and apple) of trees/shrubs affected by cankers. Cankers are a generic term meaning a sunken dead area on the stem or roots. Cankers can be caused by mechanical injury, winter injury, fungi, and bacteria among other things. You can’t always tell the exact cause from looking at a picture. To determine the cause, I would need to put it in a moist chamber and then look for fruiting bodies or evidence of bacteria under the microscope. In the case of some cankers like Nectria canker (lots of little orange fruiting bodies), the pathogen comes in on winter-injured tissue. Bottom line is, there is no cure for cankers. You have to prune out the branch with the canker and destroy. In the case of the mulberry and the elderberries that were riddled by lots of small cankers, the grower may have to cut at the base and let the plant regrow.

As always, happy to look at pictures (ann.hazelrigg@uvm.edu) or samples at the Plant Diagnostic Clinic at 63 Carrigan Drive, Burlington, VT 05405.