Survey and pathogenicity of gummosis associated oomycetes isolates and sensitivity of Alternaria isolates to fungicides from southeastern Minas Gerais
Diseases such as gummosis, mainly associated with Phytophthora species, and Alternaria Brown Spot (ABS), caused by Alternaria spp., holds a threat against the development of the orchards. Despite their importance, information regarding pathogen diversity, aggressiveness and fungicide sensitivity remains limited in the ‘Zona da Mata’ region. Therefore, this study aimed to perform a molecular characterization and assess the aggressiveness of oomycetes isolates associated with gummosis in mandarin orchards from the ‘Zona da Mata’ region, Minas Gerais, Brazil, as well to assess the sensitivity of Alternaria spp. isolates to QoI and DMI fungicides. Oomycetes isolates were obtained from rhizosphere soil of symptomatic plants and evaluated for pathogenicity on ‘Ponkan’ mandarin fruit and ‘Rangpur’ lime seedlings. For the molecular analysis, the isolates were identified based on the mitochondrial COI region. The pathogenicity assay showed that the isolates were pathogenic, the mean value of diameter lesion was 32.13 mm on fruit and 4.54 mm on seedlings. The molecular analysis classified the isolates in different species, such as Phytophthora nicotianae, Phytopythium vexans, Phytopythium chamaehyphon and Pythium nodosum. Alternaria spp. isolates collected from mandarin orchards were evaluated for sensitivity to pyraclostrobin, azoxystrobin and difenoconazole, using spore germination and mycelial growth inhibition assays. The results showed a higher frequency of less sensitive isolates to QoI fungicides than to difenoconazole. The absolute frequency of isolates less sensitive to pyraclostrobin and azoxystrobin were 46 and 48 out of 100 isolates, respectively. To difenoconazole, 22 of 100 isolates were classified as less sensitive. Overall, this study provides a first report of Phytopythium species associated with gummosis and showed Alternaria spp. isolates with reduced sensitivity to the most common fungicides in mandarin orchards from southeastern Minas Gerais. Keywords: Alternaria spp.. Sensitivity to fungicides. Rootstock. Oomycetes.