Powdery Mildew Can Be Added to the List of Diseases Infecting Honeycrisp

The unique attribute of powdery mildew that is often overlooked is its ability to infect the epidermis of apples very close to harvest. This is long after a grower has decided that all major diseases in the orchard have been looked after as well as can be and in a period of time when the last orchard fungicide spray is long past and harvest is well underway. Susceptible cultivars like ‘Fuji’, ‘Jonagold’ and now ‘Honeycrisp’ become very susceptible to the invasive properties of this circumglobal disease organism.

What normally transpires is that the disease infection becomes very obvious on terminal growth in late August and early September on cultivars like ‘Jonagold’. The cupped and whitish appearance of the leaves is a sure sign of infection. Sometimes the leaf margins show a red stippling in the absence of a mycelial mat, which normally blankets the lower side of the leaf. Some leaf abscission may have even occurred on varieties like ‘Golden Delicious’.

Temperature is the most important factor affecting the development of powdery mildew. Conidia will germinate at temperatures between 10 and 25°C, the optimum temperature is between 20-22°C.

Figure 3. Powdery mildew infection of ‘Honeycrisp’ fruit results in a fine network of rough lines that will result in cullage of affected fruit. Just left of the fruit are terminal leaves showing the gray felt-like patches of fungal mycelia and spores.

Apart from fungicide treatments that hold the disease in check, growers can help reduce the potential destruction by mildew by using practices that promote good air movement and light penetration into the canopy.