Some diseases are unavoidable: what to do when a dahlia comes down with leaf spot?

It is peak dahlia season right now. Dahlias are large and gorgeous, utterly enchanting, which is one of the reasons so many people love them. But I have also heard plenty of "worries" from fellow gardeners, about some of the care problems that come with dahlias. One of those problems is this: the leaves of the dahlia start growing spots from the edges inward and slowly turn black and scorched. What is going on?

Withered leaves mean disease

Based on my assessment, it is very likely leaf spot. No doubt many of you are not clear on what leaf spot is, or what its symptoms are. So that next time you run into the same symptoms you can go straight to the right treatment.

When a plant comes down with leaf spot, circular or semicircular dark green ring-shaped lesions appear on the leaf surface and leaf margins. After a while, the lesions begin to turn dark brown, and the centers become grayish green. Once they develop to a certain point, tiny black dots grow on the lesions, and in severe cases the leaves begin to wilt and droop.

Fungal infection is the root cause

The pathogen behind leaf spot is Phoma dahliae, a fungus in the class Deuteromycetes. The pathogen overwinters as mycelium and pycnidia on diseased plant debris or on infected plant parts. When the next year comes, it cannot be wiped out, springing back to life with the spring breeze as conidia ride the wind and rain, drifting and spreading everywhere to cause primary and secondary infections, until finally the plant falls ill.

Prompt treatment is the right answer

Fellow gardeners must not go soft on leaf spot. Promptly remove diseased plant debris or infected parts, and you can bury them deep in one place or burn them to prevent the pathogen from multiplying.

When only a few plants are affected, cutting off and burning the diseased leaves can hold back the spread of the pathogen.

Fellow gardeners, pay attention to sensible fertilizing: apply less nitrogen fertilizer and more potassium fertilizer, and step up care and management, which can improve the plant's disease resistance.

Fellow gardeners can plant at a sensible density and drain promptly, creating a growing environment with good airflow and light and no standing water, which can reduce the incidence of disease.

Once it is spotted in the early stages, you can use a 75% chlorothalonil wettable powder solution diluted 500 times, applied about once every ten days, for one or two treatments.