DETECTION OF LATENT INFECTIONS IN APPLE FRUIT WITH PARAQUAT

This research was conducted to assess the effect of paraquat on the br eakdown of apple fruit (pre- and postharvest), to identify the fungi r ecovered from paraquat-treated fruit, and to determine the potential u se of paraquat in a rapid quantitative measure of the pathogenic compo nent of fruit storability. In 1992, a greater incidence of Golden Deli cious and Nittany apple fruit developed acervuli of Colletotrichum acu tatum, conidiophores of Alternaria alternata, and pycnidia of Botryosp haeria dothidea after treatment with paraquat than without treatment. In 1993, fungi observed on both cultivars were C. acutatum, B. dothide a, Phoma spp., Phyllosticia solitaria, Penicillium expansum, and A. al ternata. Treatment of asymptomatic fruit sections with paraquat facili tated the detection of only B. dothidea, Phoma spp., and P. solitaria on Golden Delicious fruit. Exposure of Nittany fruit to paraquat facil itated the detection of B. dothidea, P. expansum, Phoma spp, and P. so litaria, but not that of C. acutatum or A. alternata. Golden Delicious fruit inoculated with B. dothidea or C. acutatum, harvested when asym ptomatic, and treated with paraquat, yielded 80 and 20% infection, res pectively, compared with only 6.7 and 0%, respectively, for the untrea ted controls. Following exposure to paraquat, naturally infected sympt omatic Golden Delicious fruit exhibited signs of B. dothidea, P. expan sum, A. alternata, Phoma spp., and P. solitaria. The incidence of C. a cutatum on paraquat-treated fruit was positively correlated with the i ncidence after cold storage (r = 0.98) and after cold storage followed by a 4-week incubation at 22 +/- 2 degrees C (r = 0.79). The incidenc e of B. dothidea on paraquat-treated fruit was not correlated with the incidence after cold storage; however, there was a positive correlati on after fruit removed from cold storage were incubated at 22 degrees C for 4 weeks (r = 0.95). The incidences of P. expansum and A. alterna ta after paraquat treatment were correlated with their incidences afte r only cold storage (r = 0.81 and r = 0.85, respectively). The inciden ces of Phoma spp. and P. solitaria on paraquat-treated fruit were not correlated with their respective incidences after any storage and incu bation conditions.