Hemileia vastatrix

Hemileia vastatrix is a fungus of the order Uredinales that causes Coffee rust, a disease that is devastating to coffee plantations.

Morphology
The mycelium with uredinia looks yellow-orange and powdery, and appears on the underside of leaves as points ~0.1 mm in diameter. Young lesions appear as chlorotic or pale yellow spots some millimetres in diameter, the older being a few centimetres in diameter. Hyphae are club-shaped with tips bearing numerous pedicels on which clusters of urediniospores are produced.

Telia are pale yellowish, teliospores often produced in uredinia; teliospores more or less spherical to limoniform, 26-40 × 20-30 µm indiameter, wall hyaline to yellowish, smooth, 1 µm thick, thicker at the apex, pedicel hyaline.

Urediniospores are more or less reniform, 26-40 × 18-28 µm, with hyaline to pale yellowish wall, 1-2 µm thick, strongly warted on the convex side, smooth on the straight or concave side, warts frequently longer (3-7 µm) on spore edges.

Spermogonia and aecia are unknown.

Life-cycle
Hemileia life-cycle begins with the germination of uredospores through germ pores in the spore. It mainly attacks the leaves and is only rarely found on young stems and fruit. Appressoria are produced which in turn produce vesicles from which entry into the substomatal cavity is gained. Within 24-48 hours, infection is completed. After successful infection, the stomatal cavity is colonized and sporulation will occur. One lesion produces 4-6 spore crops over a 3-5 month period releasing 300-400,000 spores.

Probably it does not complete its life-cycle on Coffee tree, but lives on some stage on some other, as yet unknown plant (heteroecious life-cycle).

Ecology
Hemileia vastatrix is an obligate parasite, that lives mainly on the plants of genus Coffea, reportedly also on Gardenia in South Africa. It needs suitable temperatures to develop (not less than 10°C and not greater than 35°C). The presence of free water is required for infection to be completed. Loss of moisture after germination has been initiated inhibits the whole infection process.

Sporulation is most influenced by temperature, humidity, and host resistance. The colonization process is not dependent on leaf wetness, but is influenced greatly by temperature and by plant resistance. The main effect of temperature is to determine the length of time for the colonization process (incubation period).

Hemileia vastatrix has two fungal parasites, Verticillium haemiliae and Verticillium psalliotae.

The fungus is probably of East African origin, but nowadays widely spread in Africa, tropical Asia, Middle- and South-America. Uredospores are disseminated to long distances mainly by wind, and over short distances by both wind and rain. Other agents such as animals, mainly insects and man, occasionally have been shown to be involved with dissemination.