Powdery mildew is one of the more easily recognizable plant diseases. The fungus is seen on the leaf surface as a white powdery growth; however, it may go unnoticed on lower and shaded leaves.
Be watchful of powdery mildew disease on herbs at this time of the year. Recently, an outbreak of powdery mildew was observed on greenhouse-grown rosemary plants. Large temperature fluctuations of warm days and cool nights with high relative humidity favors powdery mildew development. Unlike other fungal foliar pathogens that need prolonged periods of leaf wetness, powdery mildew is inhibited by wet leaves. Drier, but humid conditions, with relative humidity levels above 75% is required for powdery mildew fungi to infect and produce new spores.
Powdery mildew fungi are obligate pathogens, meaning they grow in and absorb nutrients
from living cells. This is one of the reasons that powdery mildew infection does not cause
necrotic lesions like other fungal leaf spot pathogens. Rather, powdery mildew grows
inside the host and often causes yellowing or purpling patches similar to nutrient
deficiency symptoms until conditions are favorable for fungal hyphal and spore growth
and development. It is a network of hyphae and single-celled spores produced in chains
that give powdery mildew its distinctive white, powdery appearance on infected plant
surfaces. Most often, powdery mildew is seen on leaves, but it can also infect stems, flowers, and fruit. Heavily infected rosemary leaves turn brown, shrivel, and drop from the plant.
Powdery mildew spores are primarily spread by wind but can also be water splashed. Infection and development of the new spores and hyphae can be rapid under favorable environmental conditions. Growers and pest managers should scout for powdery mildew weekly in the spring and fall. Look on both the upper and lower sides of the leaves for the white, powdery growth. Concentrate in areas where temperature within the greenhouse may fluctuate more readily, such as near vents and doors.
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