The length of the median incubation period of Monilinia fruticola (Wint.) Honey was determined on fruits of several Prunus species at various maturity, injury, and inoculum levels; the median incubation period being defined as the interval between application of conidia to fruits and the first appearance of sporodochia on 50% of fruits. Results were expressed as the time required for 20% and 50% of fruits to produce sporodochia. The following points were demonstrated: (1) Differences in the median incubation period between varieties for similar inoculum dosage were greatest when fruits were least matured and non-injured. (2) These differences became less apparent as inoculum dosage increased, as fruits matured, and as skin injuries increased. (3) Green fruits could be infected by high inoculum dosages when non-injured and by low inoculum dosages when injured. (4) Ripe fruits could be infected by low inoculum dosages whether non-injured or injured. (5) Free ("precipitated") moisture was necessary for infection. High relative humidity (R.H.) maintained this moisture after application of spore suspensions, thus permitting infection, whilst low R.H. allowed this moisture film to dry out, preventing infection. (6) After infection occurred, the R.H. level did not appear to affect the sporulation intensity.The above results are discussed in relation to the concept of inoculum potential, as defined by Garrett (1956), and to the development of epiphytotics and other aspects of the epidemiology of the brown rot fungus.

An investigation into the role of water relations in determining conidial regeneration of Monilinia f1"Ucticola (Wint.) Honey on infected leaves, peduncles, and mummified fruitlets of apricot (Prunus armeniaca L. cv. Trevatt) demonstrated the following points:

CORB~N, J. B., and J. M. OGAWA. 1974. Springtime dispersal patterns of Motrilitria laxaconidia in apricot, peach, prune, and almond trees. Can. J. Bot. 52: 167-176. Numbers of airborne Motrilirria 10x0 conidia were related to the maturity of the sporodochia, the nature of the host structure bearing them, and the presence or absence of rain or fog. Hirst spore traps collected more airborne conidia during rains and on rainy days than on dry days among prunaceous hosts during the spring of 1966 and 1967. Numbers of conidia were greatest (6440 m-3 h-I) in the first 3-4 h of rain, while in dry periods, numbers were greatest (1200 m-3 h-I) when ambient humidity was lowest and temperature and wind speeds were highest. Fruit mummies, twig cankers, limb cankers, pedicels, and peduncles all bore sporodochia. Rains washed off successive crops of conidia, and with 52 800 conidia ~m -~ were collected directly below a sporulating peach fruit mummy.