Whereas healthy plants are often able to ward off an infection or recover on their own, stressed plants that are growing in unsuitable locations or cared for inadequately or improperly may fall prey to this fungal disease more easily. Infection is common in crowded plantings that have poor air circulation, and in areas that are shaded and damp. While the spores are often spread by the wind, insects such as woolly aphids can spread the infection as they feed on plants. Therefore, powdery mildew is often a problem in the spring and fall, when there are large differences in temperatures between day and night in many regions. Moderate temperatures in the 70 to 80☏ range favor infection. Spore formation is favored by high humidity, while the dispersal of spores is accelerated when the humidity is low. These fungi cause the most severe infections when the humidity is high at night and low during the day. However, water in the air (humidity) is necessary for the spores to germinate. In fact, free water can kill the spores of most types of fungi that cause powdery mildew, and inhibit the growth of the mycelia. Unlike most fungal pathogens, all species that cause powdery mildew can infect their host in the absence of free water. Cucumbers and zucchini, on the other hand, may both be infected by the same species of fungi. In other words, just because the field of barley next door is exhibiting signs of powdery mildew doesn’t mean your roses will succumb to it as well. These fungi won’t attack any plant in the area like many other pathogens that assault a wide variety of species (Anthracnose and white mold come to mind). These root-like structures are called haustoria.īecause of this specialized relationship, most powdery mildews are specific for their hosts. While the fungal threads of almost all species grow on the surface of leaves, these fungi send out specialized structures that live within the plant cells to siphon their nutrients. Powdery mildew is common wherever cucurbits are grown, so vertical gardening is often recommended to improve airflow. Some common types include Podosphaera that infects rosesand Golovinomyces that attacks cucurbits. These fruiting bodies used to be called cleistothecia but are now designated as chasmothecia. Spores from the asexual stage spread the disease, while the sexual stage produces fruiting bodies that are important for overwintering. These stages look quite different from each other – to the extent that there are different scientific names for each of the stages. Like many fungi, these pathogens reproduce both sexually and asexually. The taxonomy of fungi in this order were revised extensively based on new DNA sequence data in the early 2000s, so you may sometimes see a current as well as an old name for the type of fungi known to cause this disease in a given plant.
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