DETECTION AND SIGNIFICANCE OF SYMPTOMLESS LATENT INFECTIONS OF MONILINIA-FRUCTICOLA IN PLUMS

Immature European plums with symptomless latent infections were detected in Niagara orchards which showed evidence of blossom blight or brown rot of immature fruits. Plums were surface-disinfested in 70% ethanol for 10 sec and in 0.5% NaOCl with 0.05% Tween 20 for 4 min, followed by a sterile water rinse. Fruits were dipped in paraquat (Pq) (6 g/L) for 1 min followed by a 3 min sterile water rinse, or ethephon (Eth)(1.5 g/L) for 4 min without a rinse. Treated plums were incubated in sterile containers at high humidity (>95%) at 25-degrees-C in light (L) or darkness (D). Fruits at the pit-hardening stage of development responded rapidly to the Pq-L treatment, with 80% of them developing brown rot after 5 days of incubation. The order of descending efficacy was: Pq-L, Pq-D, Eth-D, Eth-L, Check-D, Check-L. For Stanley and Bluefre plums at the fruit-coloration stage, latent infections responded similarly, and their rate of development into brown rot lesions with sporodochia was greater for a high incidence than for a low incidence of latent infection. The combined percentage of plums with brown rot at harvest and after postharvest incubation, were positively correlated with the percentages of immature fruits with symptomless latent infection [r = 0.917, (P = 0.01) and r = 0.954 (P = 0.001) for 7 and 14 days incubation, respectively]. The earlier detection of latent infections would allow corrective measures to be taken preharvest or postharvest to reduce brown rot of mature plums.