Germination and penetration studies on coffee rust (Hemileia vastatrix B. & Br.)

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Abstract
SUMMARY: Spore dispersal, germination, penetration and incubation‐period studies on coffee rust (Hemileia vastatrix B. & Br.) in Kenya, East Africa, are described.Evidence is produced that air‐borne spores may be trapped effectively on the upper surfaces of leaves, thence to be liberated and transported to the undersurfaces of other leaves by rain splash.Germination requires liquid water and was observed to occur in 2.6–4.7 hr‐(medians) at 23° C., the minimum being 1 hr. Appressoria were formed in 6.5–8.5 hr. (medians) with a 5.3 hr. minimum. Germination is inhibited by light and in the field by the rapid evaporation of water droplets on the lower leaf surface which occurs during daylight. Light inhibits the growth of germ tubes that are less than 30 μ long and reduces rate of growth if they are longer; appressoria may continue to form.In the field, appressorium formation and infection can occur between 10 p.m. and 8 a.m. If coffee trees are wet at dusk or rain falls before midnight, infection is probable and it is proposed that the number of occasions this occurs be used to forecast the severity of the annual rust maximum.The median incubation period throughout the year varied from 4 to 7 weeks, increasing with low temperatures and dry conditions. A multiple regression of mean maximum and minimum temperatures on incubation period gave fair agreement between observed and computed values for Ceylon and Mysore. Susceptibility and incubation period were strongly affected by coffee variety and rust biotype, but not by age of leaf or crop size.

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