Sensitivity of Lasiodiplodia theobromae from Brazilian papaya orchards to MBC and DMI fungicides

Stem rot caused by Lasiodiplodia theobromae is an important postharvest disease of papaya in Brazil, responsible for reducing the quality and quantity of fruits. Fungicide use is one of the main disease management measures. However, there are no estimates available of pathogen sensitivity to commonly employed fungicides. Therefore, the EC50 from 120 isolates of L. theobromae from northeastern Brazil, representative of six populations of the pathogen, was estimated in vitro for fungicides of the methyl benzimidazole carbamates-MBC (benomyl and thiabendazole) and demethylation-inhibiting-DMI (imazalil, prochloraz, tebuconazole) groups. Mycelial growth on fungicide-free media and virulence on papaya fruits of the MBC-sensitive and non-sensitive isolates were compared. For MBCs, 8.4% of isolates were non-sensitive to fungicides. For the remaining 91.6%, the mean EC50 ranged from 0.002 to 0.13 mu g ml(-1) and 0.36 to 1.27 mu g ml(-1) for benomyl and thiabendazole, respectively. For DMIs, the mean EC50 range for imazalil was 0.001 to 2.27 mu g ml(-1), 0.04 to 1.75 mu g ml(-1) for prochloraz, and 0.14 to 4.05 mu g ml(-1) for tebuconazole. The EC50 values of non-sensitive isolates were significantly (P <= 0.05) higher those for the sensitive isolates for each of the DMI fungicides. Differences (P <= 0.05) were found in the levels of sensitivity to DMI fungicides among the isolate populations associated with orchards. The populations from two orchards were less sensitive to DMIs. No solid evidence was found for fitness costs relating to MBC non-sensitive isolates because mycelial growth in fungicide-free media and virulence on papaya fruits were similar to those of sensitive isolates.