Detection of Brasiliomyces malachrae Causing Powdery Mildew on Ornamental Cotton (Gossypium hirsutum) plants in Mexico.

Cotton (Gossypium L.; Malvaceae) is the most important fiber crop worldwide, also as a source of vegetable protein and edible oil. Cultivated species of cotton were apparently domesticated independently in four separate regions, in both the Old and the New World. Due to its economic importance, it is necessary to study the diseases that limit its production. During July of 2020-2022, symptoms of powdery mildew were observed on 80 ornamental cotton plants in a nursery located in Cuautla (18°52'38"N; 98°58'28"W), Morelos, Mexico. Disease incidence was 29%. Signs first appeared as small white colonies, which subsequently developed into abundant mycelial grown mainly on the upper leaf surface. White patches of mycelia were observed on leaves. In advanced stages of the disease, plants exhibited symptoms of yellowing, necrosis, and early defoliation. Microscopic analysis from 10 plant samples showed that mycelia were amphigenous, epiphyllous, in thin patches and evanescent. Hyphae were hyaline, thin walled and hyphal appressoria were simply lobed. Chasmothecia (n=50) were sub-aggregate, generally spherical to subglobose (46-61 µm in diameter), whitish, subhyaline, smooth, with a peridium of a single cell layer and appendages were absent. Three asci per chasmothecia, subspherical, 30-44 × 26-38 µm, with 4-6 ascospores per ascus. Ascospores were hyaline, ellipsoid to ovoid (16-23 × 10-18 µm). The asexual phase was not observed. The characteristics observed correspond to Brasiliomyces malachrae (Braun and Cook 2012; Cabrera et al. 2018). A voucher specimen was deposited in the Herbarium of the Department of Plant-Insect Interactions at the Biotic Products Development Center of the National Polytechnic Institute under accession no. IPN 10.0114. To confirm identification, DNA was recovered from the fungus and the internal transcribed spacer (ITS) from one sample was amplified by PCR, using the primers ITS1/ITS4 (White et al. 1990). The sequence was deposited in GenBank (OQ546720) and showed 100% sequence homology (647/1642bp) with the type sequence of B. malachrae (LC191217) from Malvastrum coromandelianum in Argentina (Cabrera et al. 2018). Pathogenicity was verified through inoculation by gently dusting conidia from infected leaves onto leaves of five healthy cotton plants. Five noninoculated plants served as controls. All plants were maintained in a greenhouse at temperatures from 28±2°C and relative humidity ranging from 80±5%. The experiment was performed twice. Inoculated plants developed powdery mildew symptoms after 14 days, whereas the control plants remained healthy. The fungus on the inoculated leaves was morphologically identical to that originally observed on diseased plants, thus fulfilling Koch's postulates. To our knowledge, this is the first report of Brasiliomyces malachrae causing powdery mildew on Gossypium hirsutum in Mexico and North America (Farr and Rossman 2023). Powdery mildew on G. hirsutum caused by B. malachrae has been previously identified in Venezuela by Hanlin and Tortolero (1984). This disease could be a primary source of inoculum of powdery mildew for commercial cotton plantations, derived from the free movement of ornamental plants.