Detection of Latent Infections in Apple Fruit with Paraquat

Abstract

Biggs, A. R. 1995. Detection of latent infections in apple fruit with paraquat. Plant Dis. 79:1062-1067. This research was conducted to assess the effect of paraquat on the breakdown of apple fruit (pre- and postharvest), to identify the fungi recovered from paraquat-treated fruit, and to determine the potential use of paraquat in a rapid quantitative measure of the pathogenic component of fruit storability. In 1992, a greater incidence of Golden Delicious and Nittany apple fruit developed acervuli of Colletotrichum acutatum, conidiophores of Alternaria alternata, and pycnidia of Botryosphaeria dothidea after treatment with paraquat than without treatment. In 1993, fungi observed on both cultivars were C. acutatum, B. dothidea, Phoma spp., Phyllosticta solitaria, Penicillium expansum, and A. alternata. Treatment of asymptomatic fruit sections with paraquat facilitated the detection of only B. dothidea, Phoma spp., and P. solitaria on Golden Delicious fruit. Exposure of Nittany fruit to paraquat facilitated the detection of B. dothidea, P. expansum, Phoma spp, and P. solitaria, but not that of C. acutatum or A. alternata. Golden Delicious fruit inoculated with B. dothidea or C. acutatum, harvested when asymptomatic, and treated with paraquat, yielded 80 and 20% infection, respectively, compared with only 6.7 and 0%, respectively, for the untreated controls. Following exposure to paraquat, naturally infected symptomatic Golden Delicious fruit exhibited signs of B. dothidea, P. expansum, A. alternata, Phoma spp., and P. solitaria. The incidence of C. acutatum on paraquat-treated fruit was positively correlated with the incidence after cold storage (r = 0.98) and after cold storage followed by a 4-week incubation at 22 ± 2°C (r = 0.79). The incidence of B. dothidea on paraquat-treated fruit was not correlated with the incidence after cold storage; however, there was a positive correlation after fruit removed from cold storage were incubated at 22°C for 4 weeks (r = 0.95). The incidences of P. expansum and A. alternata after paraquat treatment were correlated with their incidences after only cold storage (r = 0.81 and r = 0.85, respectively). The incidences of Phoma spp. and P. solitaria on paraquat-treated fruit were not correlated with their respective incidences after any storage and incubation conditions.