Hormetic-like effect and management of thiophanate-methyl resistance in Lasiodiplodia theobromae isolates

Stem-end rot caused by Lasiodiplodia theobromae is one of the main diseases in papaya crop (Carica papaya). Resistance to the fungicide thiophanate-methyl (TM) has been found in isolates of this pathogen in the northeast of Brazil. The goals of this study were to evaluate the sensitivity of TM resistant and sensitive L. theobromae isolates to other fungicides, and to investigate the hormetic effect of fungicides that could be used as alternatives to manage the disease. In vitro and in situ assays were conducted, using fungicides registered and not registered for the management of papaya diseases Imazalil was the most effective in controlling stem-end rot. Thiabendazole, a fungicide registered to prevent the disease post-harvest, proved ineffective against a resistant isolate and significantly increased its virulence by 47% at the recommended therapeutic dose. Additionally, subinhibitory doses of TM lead to stimulation of mycelial growth by 31% over the control in a typical hormetic-like response. Not registered fungicides stimulated the pathogen’s virulence. The resistant isolate was significantly stimulated by trifloxystrobin (15%), kresoxim-methyl (24%); while the sensitive isolate was significantly stimulated only by kresoxim-methyl (47%). These observations suggest the need for new strategies to manage the disease in areas with a history of resistance, especially into an anti-resistance strategy scenario. Imazalil (postharvest), for this crop is a good option to control L. theobromae isolates and the occurrence of stimuli induced by commercial and subinhibitory doses of fungicides is a potential problem that has to be thought in a program of management of the disease.