Root pathogens in cucurbits: species diversity, pathogenicity and chemical control
Authors:   , Alves Cynthia Patricia de Sousa Santos 
Published: December 2024

The genus Macrophomina and Lasiodiplodia are associated with root rot and vine decline in cucurbit crops in the semiarid region of Northeastern Brazil, causing economic losses. Little is known about the diversity, pathogenicity and management of fungal species in these production areas. To address these issues, three studies were conducted, with the aim of identifying the genetic diversity and pathogenicity of fungi established in areas producing watermelon for export and studying the effect of different active ingredients and doses on the in vitro and in vivo mycelial growth of Macrophomina spp. in melon plants. In the first study, 30 symptomatic watermelon plants were collected in each of the 16 commercial watermelon production fields evaluated, in the states of Rio Grande do Norte and Ceará. From each plant collected, samples of the root part were isolated for fungal isolation and subsequent identification at the genus level. A total of 156 isolates were molecularly identified using DNA extraction and polymerase chain reaction amplification, with specific primers (MpTefF and MpTefR for M. phaseolina, MsTefF and MsTefR for M. pseudophaseolina, and MeTefF and MeTefR for M. euphorbiicola). To confirm the pathogenicity, 50 isolates of M. phaseolina and 50 of M. pseudophaseolina were randomly selected. Disease incidence and severity, shoot and root length and dry weight of all plants in the trial were evaluated. All Macrophomina isolates tested were observed and confirmed as pathogenic for watermelon, with M. phaseolina showing higher incidence (97%) and severity (3.21) of the disease compared to M. pseudophaseolina (70% and 1.15, respectively). This was the first pathogenic report of M. pseudophaseolina on watermelon in the world. Watermelon trees infected with M. phaseolina corresponded to greater shoot (74.89 cm) and root (17.96 cm) length, and lower dry weight (2.12 g) compared to M. pseudophaseolina (66.61 cm, 17.42 cm and 2.22 g, respectively). The second study evaluated the in vitro and in vivo effect of boscalid, carbendazim, cyprodinil, fluazinam and fludioxonil in five doses (0.01; 0.10; 1; 10; and 100 mg/L a.i.) on daily mycelial growth, in percentage of inhibition and effective concentration to reduce 50% of mycelial growth of nine Macrophomina isolates (M. phaseolina: CMM1556, CMM4748 and CMM4764; M. pseudophaseolina: CMM2163, CMM4815 and CMM4767; and M. euphorbiicola: CMM2158, CMM4868 and CMM4867). Results of in vitro tests showed that fluazinam and fludioxonil were highly toxic (EC50 = 0.03 mg/L a.i.) to Macrophomina. In in vivo tests, fluazinam was able to reduce the pathogenic damage of M. phaseolina and M. pseudophaseolina in melon, by at least 21.43%. However, M. euphorbiicola isolates caused lower disease incidence (28.57%) and severity (0.29) in plants treated with fludioxonil. The third study reported for the first time in Brazil and in the world the occurrence of Lasiodiplodia brasiliensis in watermelon. Pathogenicity tests confirmed that the isolates caused characteristic symptoms of decline in watermelon