First Report of Eggplant Fruit Rot Caused by Phytophthora nicotianae Breda de Haan in Mexico.

Abstract

Eggplant (Solanum melongena L.) is an important solanaceous crop that is produced mainly in tropical and subtropical regions and is widely consumed worldwide. In 2018, eggplant production in Mexico was approximately 80000 t, and Sinaloa State contributed 96% of this production; however, this crop suffers significant losses from plant pathogens. In December 2019, fruits from commercial orchards (geographical coordinates: 24°45'39.39''N, 107°26'57.30''O) with visible brown soft rot and profuse white mycelia were analysed. On V8 medium, pieces of tissue obtained from the border of lesions were plated and incubated between 25°C and 36°C. After five days, a dense cottony mycelium with a slightly petaloid pattern was observed at 25°C and did not grow at 36°C. Isolates of that pathogen were heterothallic, and microscopic preparations showed development of coenocytic mycelium and spheroid sporangia that were noncaducous and papillate, measuring 35.6 ± 5.8 x 27.1 ± 4.4. Based on morphological characteristics, the eggplant soft rot causal agent was identified as Phytophthora nicotianae Breda de Haan (Erwin and Ribeiro, 1996). From a representative isolate denominated PhySm01, two DNA regions (internal transcribed spacer (ITS) and the large subunit ribosomal (28S)) were amplified and sequenced with ITS1-ITS4 and NL1-LR3 primers, respectively. The obtained ITS sequence (GenBank accession number MT508842) showed 100% identity with several P. nicotianae sequences (Access MT065840, MH290435 and MG570057) with 100% query coverage and 740 matching nucleotides. For the 28S sequence (accession number MT508843), the identity with strains N° Access EU080889 and EU080508 of P. nicotianae was 99.86%, with 100 query coverage and 729 matching nucleotides. Further, phylogenetic analysis from P. nicotianae strain PhySm01 and GenBank reference sequences was carried out by Maximum Likelihood method with Mega 7 software based on the ITS sequences, which verified the species identification. To fulfill Koch's postulates, a suspension containing 1 x 104 zoospores/mL of the oomycete isolated from the original diseased eggplant fruit was used to inoculate ten healthy and disinfested fruits. Sterile water was used as a control. Three wounds per fruit were made with a sterile needle, and 20 μL of the zoospore suspension (or water) was placed on each. All fruits were placed into plastic bags with moistened paper and incubated at 25°C for three days. Thisest was repeated twice with similar results. Initial symptoms developed 24 hours after inoculation with brown soft tissue forming around the inoculated area, and profuse soft rot accompanied by white mycelium was observed two days after inoculation. No symptoms developed on the control fruits during this time. Pieces of necrotic tissue were plated on V8 medium and incubated as described previously. The reisolated pathogen was compared to the original isolate and had the same morphological characteristics. Phytophthora nicotianae has a worldwide distribution and can infect multiple solanaceous crops, including tobacco (Gallup et al., 2018). In addition, it has been reported on other hosts outside of the Solanaceae family, causing economically important losses in citrus and strawberry. In India, it causes necrosis of citrus fruits and roots, leading to tree decline (Das et al., 2016), and in the United States, it causes crown rot of strawberry (Marin et al., 2018). In Mexico, P. nicotianae has been reported on vinca (Alvarez-Rodriguez et al., 2013); however, this is the first report of P. nicotianae causing fruit soft rot of eggplant, and this pathogen represents a new threat when the environment is favourable for disease outbreaks. References Alvarez-Rodriguez, B. et al. 2013. Plant Dis. 97: 1257. https://doi.org/10.1094/PDIS-04-13-0400-PDN Das, A. K. et al. 2016. J. Plant Pathol. 98: 55. DOI: 10.4454/JPP.V98I1.038 Erwin, D. C. and Ribeiro, O. K. 1996. Phytophthora Diseases Worldwide. The American Phytopathological Society, St. Paul, MN. 562 Pp. Gallup, C. A. et al. 2018. Plan Dis. 102: 1108. https://doi.org/10.1094/PDIS-02-17-0295-RE Marin, M. V. et al. 2018. Plant Dis. 102:1463. https://doi.org/10.1094/PDIS-08-17-1333-PDN.