Cucumber plants turning yellow, bumps on grape leaves and early shedding on a tree
Q: I have attached a few pictures of the cucumber plants that are turning yellow, and a few leaves have started to dry up. The vines have a lot of blossoms. The variety is Straight 8 and was planted on May 18. — Chris Levorsen, Fargo.
A: The cucumber leaves are likely being affected by a fungus disease called anthracnose. An all-purpose garden fungicide, containing the active ingredient chlorothalonil, which can be found in fine print on the ingredient label, can be applied as a preventative to limit the spread of the disease to new and unaffected foliage.
Fungi are often spread by overhead sprinkling, which spreads the disease organisms from the soil to the lower leaves and upward. If possible, water only the soil and avoid overhead sprinkling. Water in morning, so any water that splashes on leaves dries quickly. Watering in the evening causes leaves to remain wet longer, increasing the chance of disease.
Next year you might look for a cucumber variety that lists disease resistance. Varieties vary greatly in their ability to fight disease, and Straight 8 is a wonderful older variety, but it has limited disease resistance.
Q: I have a grape vine with a problem. The grapes themselves look good, but the leaves have some sort of growth on them. Do you have any ideas? — Linda Gregg, Horace, N.D.
A: From your photo, the grape leaves have what are generically called leaf galls, formed by small mites or other pests, and although they look scary, they usually have little or no adverse effect on the health of the leaves, vines or fruit.
By the time the galls are noticed, the mites are protected inside the gall, and any insecticides applied have little effect. Leaf galls are generally considered cosmetic, without need to control.
However, if one would like to control the mites, the preferred time is in spring, just as the buds are starting to open. At that time, apply horticultural oil to the vines and trunks. Horticultural oil is available at garden centers. As the mites crawl up the vines or trunks, they are killed or inhibited by the oil.
Q: We have a mature flowering crab tree in the backyard. It is a beautiful tree, but for the last couple of weeks, the leaves are turning yellow or brown and dropping off the tree. Is there something that should be done to stop the early shedding of leaves? — Dean Badinger, Fargo.
A: From your description, the flowering crab is likely showing the symptoms of the fungus disease called apple scab, which affects both fruiting apples and ornamental flowering crabs. The disease is worse in summers of high humidity, and I’ve noticed a fair amount of apple scab disease around the region.
Different cultivars of flowering crabs vary greatly in susceptibility, which is why some types are affected and others aren’t. The long-term health of the tree isn’t greatly affected by the disease, as it happens in late summer when the leaves will soon be shed naturally, but in the meantime it makes the tree’s canopy look thin.
The best control, other than resistant cultivars, is an application of fungicide applied before the symptoms appear, especially following periods of heat and humidity. Prevention works best, as once the tree is affected, it’s difficult to contain the disease for that season. All-purpose fungicides containing the active ingredient chlorothalonil can be applied in midsummer while the foliage still looks healthy. Raking or bagging fallen leaves of affected trees can help reduce future problems.