Effects of infection by Phytophthora injestans (Mont.) de Bary on the stomata of potato leaves

Phytophthora injestans infection of potato leaves causes abnormal opening of the stomata in the tissue colonized by the fungus before sporangiophores emerge through them. The affected area, which may be up to 7 mm wide, extends from the necrotic tissue to within 1–2 mm of the advancing edge of the colonizing mycelium. These stomata open wider than those in uninfected parts of the leaf, and do not close in the dark, but closure can be induced experimentally by a high water deficit. Affected guard cells have an increased osmotic value, reduced starch content, and show degradation of the chloro-plasts. No direct infection of the guard cells has been observed. Autoradiography of infected leaflets which had been exposed to ¹⁴CO₂ in the light, showed that a zone of increased photosynthesis occurs in the region of wide-open stomata.