Scanning electron micrograph of a germinated spore of the mildew, Erisyphe pisi, on a pea leaf, Pisum sativum. The picture shows a spore, 25 microns long (red) that first attached itself (at its right) to the leaf before producing hyphae. A leaf surface is a hostile environment; at times drenched by rain, or hot and dry, in sunlight. A spore must quickly access the leaf interior; by first developing an "adpressorium" to secure its hold, then penetrating the leaf surface to obtain water. The growth of hyphae (blue) follows. They enter the leaf at intervals, by dissolving its wall. A few scattered stomata appear in the picture; they are not points of entry here. Powdery mildews are actinomycete fungi that in dry seasons may cause serious losses to cereal or sugar beet crops. They can overcome resistance bred into crops, even evade fungicides, by developing new genetic races.