Germinated spore of mildew, Erisyphe pisi

Scanning electron micrograph of a newly germinated spore of the mildew, Erisyphe pisi, on a pea leaf, Pisum sativum. The picture shows a spore, 25 microns long (yellow) attached to the leaf at its lower left (pink). This is the site of the appressorium that secures the spore, and which pierces the leaf wall to obtain water from the interior. Once the water supply is established, the spore produces hyphae; here, so far one only, to lower right. Hyphal branches can also enter the leaf at intervals, by dissolving its wall (not shown here). These hyphal connections allow the pathogen to survive even the most hostile conditions on the surface of the leaf. Powdery mildews are actinomycete fungi that, especially 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., Photo by DR JEREMY BURGESS/SCIENCE PHOTO LIBRARY