Etiology and management of fungal root rot of cassava in Southern Kerala

Abstract

The research entitled “Etiology and management of fungal root rot of cassava in Southern Kerala” was conducted in the Department of Plant Pathology, College of Agriculture, Vellayani, during 2020-2022 with an objective of Identification and characterization of major pathogen causing fungal root rot in cassava and its management using different biocontrol agents and fungicides. During this study, root rot infected cassava plants were collected from different locations in the Thiruvananthapuram and Kollam districts of Southern Kerala. Twenty-one isolates of different fungi were obtained from root rot infected parts of cassava. Various symptoms such as yellowing of leaves, rotting of tubers and collar region of cassava stems, mycelial growth over the infected tubers and wilting of cassava plants were noticed in diseased samples from different locations. Based on the pathogenicity test conducted on potted cassava plants of one - month old Vellayani Hraswa variety, T3B (isolate from Thalayal of Thiruvananthapuram district) and K1B (isolate from Ezhukone of Kollam district) isolates were found to cause 100 Percent Disease Incidence (PDI). Based on the number of days taken for disease development, the isolate T3B (Fusarium sp.) was found to be most virulent with a PDI of 100, which took 30 days for symptom development, followed by K1B (Fusarium sp.) with PDI of 100 and 40 days for symptom development. Hence, T3B was selected for further studies. Cultural and morphological characteristics of the T3B isolate were studied. The colony was white cottony, with irregular margins on the front view and a light pinkish tinge on the rear view. Microscopic characters of the T3B isolate revealed that the mycelia were septate and hyaline. Macroconidia and microconidia were produced abundantly on conidiophores. Macroconidia were hyaline, septate (0-2 septa), slender, almost straight and elongated with blunt ends, while the microconidia were slender, hyaline, and single celled with oblong shape. Chlamydospores were absent. Molecular characterization of T3B isolate was carried out using ITS sequencing using universal primers followed by comparative nucleotide sequence alignment with the available database from NCBI. The isolate T3B was confirmed as Fusarium proliferatum with 99.79 per cent homology with F. proliferatum strain OR25F07 (MT476359.1) and the dendrogram was also constructed as a distance tree which showed a phylogenetic relationship. The nucleotide sequence of the T3B isolate was deposited in GenBank and the accession number was retrieved as OQ376433. As part of the study, microflora was isolated from the rhizosphere of healthy cassava plants from Vellayani and Venganoor of Thiruvananthapuram district in Kerala. Among the isolated bacterial and fungal rhizosphere microflora, viz., Aspergillus sp. 1, Aspergillus sp. 2 Bacteria 1 and Bacteria 2, Aspergillus sp. 1 from Vellayani showed the highest inhibitory effect against the pathogen with 64.84 per cent inhibition. The in vitro antagonism of seven biocontrol agents viz., Pseudomonas fluorescens PN026, Bacillus sp. isolate 1 and Bacillus sp. isolate 2 (RRS, Moncompu), Trichoderma sp. 1, Trichoderma sp. 2 (KAU), Piriformospora indica (No. INBA 3202001787) and Aspergillus sp. 1 (selected antagonist) were evaluated against F. proliferatum by dual culture technique and results revealed that highest percentage inhibition was recorded with Trichoderma sp. 1 (57.57 %) followed by Trichoderma sp. 2 (51.51 %). The in vitro evaluation of fungicides through poisoned food technique against F. proliferatum was conducted using eight fungicides viz., mancozeb 75% WP, copper oxychloride 50% WP, propineb 70% WP, carbendazim 50% WP, propiconazole 25 % EC, cymoxanil 8% + mancozeb 64% WP, carbendazim 12% + mancozeb 63% WP and trifloxystrobin 25%+ tebuconazole 50 % 75 WG at 50, 100 and 200 ppm. The highest per cent inhibition was showed by carbendazim 50% WP and trifloxystrobin 25%+ tebuconazole 50 % 75 WG at 200 ppm with an inhibition percentage of 87.87% and 100%, respectively. Carbendazim 50% WP and trifloxystrobin 25%+ tebuconazole 50 % 75 WG were the most effective fungicides used for in vitro assay. A pot culture study was undertaken with the Vellayani Hraswa variety to evaluate the efficacy of effective biocontrol agents and fungicides, viz., Trichoderma sp. 1. Trichoderma sp. 2 (KAU), carbendazim 50% WP and trifloxystrobin 25%+ tebuconazole 50 % 75 WG against F. proliferatum and results shown that the highest disease suppression over control was recorded with Trichoderma sp. 2 (100.00%) followed by Trichoderma sp. 1 (88.89%) and trifloxystrobin 25%+ tebuconazole 50 % 75 WG (77.78%) with sett treatment and soil application at one and two months after planting. The highest yield of 1342.66 g per plant was obtained from treatment with Trichoderma sp. 2 @108 cfu/ml followed by trifloxystrobin 25%+ tebuconazole 50% 75 WG @1 gL-1 (1106.00 g per plant) and Trichoderma sp. 1 @108 cfu/ml (1003.66 g per plant). The present study revealed that Fusarium spp., Lasiodiplodia spp., Colletotrichum spp. and Rhizoctonia spp. were associated with root rot of cassava in Southern Kerala. The most virulent pathogen was identified as Fusarium prolifaratum by analysing the cultural, morphological and molecular characteristics. The sett treatment and soil application of biocontrol agent Trichoderma sp. 2 (KAU isolate) (108 cfu/ml) or the fungicide trifloxystrobin 25%+ tebuconazole 50% 75 WG (1 gL-1 ) at one and two months after planting can effectively manage the disease.