Dilip Kumar Lakshman

Myxobacteria, a group of swarming, predatory soil bacteria, are of interest because of their biocontrol potential. In this study, the inhibitory effects of 13 strains of myxobacteria were examined against four different phytopathogenic fungi, as follows: two isolates of Rhizoctonia solani from different AG groups and one isolate each from Sclerotinia sclerotiorum and the oomycete Pythium ultimum. Inhibition levels varied among phytopathogens, with slow-growers being more susceptible than fast-growers. Myxococcus xanthus BS 248, M. flavus ATCC 29617, and M. coralloides BS249 were the most inhibitory strains tested. non-contact and contact inhibition on agar media between phytopathogens and myxobacteria were visually discernible. This distinction potentially reflects the activity of low-molecular-weight metabolites and high-molecular-weight lytic enzymes, respectively. In a pot soil study, the inhibitory effect of a mixture of two strains of myxobacteria against two strains of R. solani was apparent from the reduced disease in cucumber seedlings compared to controls without myxobacteria. This is the first report of an in vivo inhibitory effect of myxobacteria against Rhizoctonia. The survival of M. xanthus BS248 in sterile soil amended with rabbit manure (1:1) increased up to five weeks compared to one week in soil without the manure, suggesting that organic amendment could enrich myxobacteria in soil.

Plant disease resistance is often a complex, polygenic trait, making its genetic dissection with traditional genome-wide association studies (GWAS) challenging. Grain mold in sorghum, a devastating disease caused by a fungal complex, exemplifies this complexity. We hypothesized that a machine learning (ML)-driven GWAS, employing diverse phenotypic representations from a panel of 306 sorghum accessions, could more effectively unravel the genetic basis of resistance. Phenotypic data, including raw disease scores, a 'difference phenotype' (inoculated vs. control), and principal components, were analyzed using Boosted Tree and Bootstrap Forest models, demonstrating strong explanatory power for phenotypic variance when trained on the entire dataset. This ML-GWAS approach confirmed a highly polygenic architecture for grain mold resistance, identifying numerous SNPs across the sorghum genome. Notably, several SNPs were consistently associated with resistance across multiple analytical models and phenotypic representations. These robustly identified SNPs were frequently located near genes with predicted functions integral to plant defense. Gene ontology (GO) analyses of the candidate gene set confirmed enrichment in categories supporting roles in pathogen recognition, DNA repair, and stress response modulation, indicating a multifaceted defense mechanism. This study provides valuable candidate genes for breeding sorghum with enhanced grain mold resistance and offers a refined methodological framework for dissecting complex traits in this crop. The successful application of this ML-based strategy in sorghum suggests its potential utility for studying similar complex traits in other plant species.

Root rot of sugar beet (Beta vulgaris L.), caused by Rhizoctonia solani anastomosis group AG 2-2 IIIB is responsible for significant crop losses in North Dakota and Minnesota, USA. Understanding the association between plant age and inoculum density with disease sever­ity of sugar beet cultivars is a prerequisite to properly screen for varietal resistance. There­fore, investigations were conducted to determine the responses of 4-, 6-, and 8-week-old plants in seven commercial sugar beet cultivars to inoculum densities of one, two, and three grains of R. solani-colonized barley in a greenhouse and with three corresponding levels of colonized barley, mycelial plugs, and sclerotia in field experiments. Under greenhouse con­ditions, disease severity was greatest before plants reached six weeks of age (p = 0.05). There was a positive linear relationship between the density of the inoculum and disease severity. All seven cultivars were equally susceptible (p > 0.05) to R. solani. Interactions between cul­tivars and plant age and between plant age and intensity of inoculum were not significant (p > 0.05). Field experiments showed that the density of inoculums was significant (p < 0.001), and the disease severity was highest in plants inoculated with three colonized barley seeds per plant compared to doses of other inoculum types.

Sugar beets produced in the Red River Valley of Minnesota and North Dakota, U.S.A., are stored in large outdoor piles or sheds for up to 280 days before processing into sugar. During storage, roots often succumb to postharvest rots, which reduce the yield of recoverable sucrose from the roots. Penicillium spp. are commonly responsible for postharvest sucrose loss. However, for this production region, the species responsible for postharvest rots and their ability to establish infection have not been investigated in recent years. To determine the Penicillium spp. responsible for storage rot in the Red River Valley, Penicillium isolates were obtained from rotted roots that were collected from storage piles located throughout the Red River Valley in Minnesota and North Dakota after variable times in storage. Morphological characterization and molecular identification of species using the DNA sequence of the internal transcribed spacer (ITS), β-tubulin ( tub2), and RNA polymerase II second largest subunit ( rpb2) regions of the pathogen genome indicated the presence of four Penicillium species in sugar beet root piles. The four Penicillium strains were identified, in order of their prevalence in piles, as P. cellarum, P. paneum, P. crustosum, and P. polonicum. Regional variations in the distribution of Penicillium species were noticed. Inoculation of healthy roots with isolates of all four Penicillium species caused storage rot symptoms resembling those observed on storage roots from which the Penicillium species were isolated. While P. cellarum has been previously identified in U.S. storage piles, this is the first report of P. paneum and P. crustosum as causal organisms of sugar beet storage rot in the United States. These findings will be useful for the identification of the pathogens present in the storage piles and the formulation of disease management practices to control the storage rot of sugar beet.

As environmental and health concerns increase, the trend toward sustainable agriculture is moving toward using biological agents. About 60% of all biological fungicides have Trichoderma species as the active ingredient, with T. harzianum as the most common species in these products. However, the name T. harzianum has often been used incorrectly in culture collections, databases, and scientific literature due to the division of the Harzianum clade (HC) into more than 95 cryptic species, with only one being named T. harzianum. In this study, 49 strains previously identified as T. harzianum in three surveys of Trichoderma species from soils in South and Central America were re-identified using phylogenetic analyses based on tef1α, rpb2, and ITS sequences obtained from GenBank. These were combined with the HC species from two other studies, which were identified based on the current taxonomy. Based on the results of the five surveys of the total 148 strains in HC, 11 species were identified. T. afroharzianum, T. lentiforme, and T. endophyticum, followed by T. azevedoi and T. harzianum, were the dominant species of the HC in South and Central America. This is the first report to identify dominant Trichoderma species within the HC in South and Central American soil based on multiple studies. These results will be useful in selecting strains within the clade for the formulation of biocontrol and biofertilizer products on the continent.

As environmental and health concerns increase, the trend of sustainable agriculture moves toward using biological agents. About 60% of all biological fungicides have Trichoderma species as the active ingredient, with T. harzianum as the most common species in these products. However, the name T. harzianum has often been incorrectly used in culture collections, databases, and the scientific literature due to the division of the Harzianum Complex Clade (HCC) into more than 95 cryptic species with only one being named T. harzianum. In this study, 49 strains previously identified as T. harzianum in three surveys of Trichoderma species from soils in South and Central America were re-identified using phylogenetic analyses based on tef1α, rpb2, and ITS loci and were combined with the corresponding species in two other studies, where the species were correctly identified. Based on the results of the five surveys of the total 148 strains in HCC, 11 species were identified and T. afroharzianum, T. lentiforme, T. endophyticum followed by T. azedevio and T. harzianum were detected as the dominant species of the HCC in South and Central America. This is the first report of identifying dominant Trichoderma species within the HCC in South and Central American soil based on multiple studies, which will be useful in selecting strains within the complex clade for formulation of biocontrol and biofertilizer products in the continent.

Fusarium stalk rot (FSR), caused by the Fusarium species complex, is an economic threat to maize cultivation all over the world. We investigated the population structure and genetic diversity of Fusarium species obtained from five major maize-growing regions in India. The Tef-1α locus was used for phylogenetic analysis of geographically distinct isolates of Fusarium verticillioides, F. andiyazi, F. proliferatum, F. nygamai, and F. acutatum causing FSR. A phylogenetic tree showed monophyletic, polyphyletic, and paraphyletic groupings reflecting the complex evolutionary history and genetic diversity within the genus. Monophyletic groupings depicting strong bootstrap support were shown to have a single common ancestor and genetic coherence with limited genetic divergence among sequences. Polyphyletic groupings also presented significant genetic differentiation within the F. verticillioides sequences from diverse ecological niches. Nucleotide diversity of moderate level 0.02471 reflected genetic variations within populations that were attributed to factors such as mutation, genetic drift, or varying selection pressures. The Fst value of 0.98205 is particularly indicative of high genetic differentiation, implying that most of the genetic variance is due to differences between populations rather than within them. F. verticillioides, with 57 sequences, showed low genetic diversity with three segregating sites and a low haplotype diversity of 0.19486, suggesting the founder effect, where a reduced population expands from a limited genetic pool. The total data estimates across all populations for haplotype analysis showed 72 sequences, 44 segregating sites, and 9 haplotypes with a haplotype diversity of 0.48513. The evolutionary dynamics showed genetic differentiations among Fusarium species causing FSR. AMOVA indicated high within-population variations, depicting a substantial genetic diversity within individual populations. The results offer a comprehensive framework for discussing the implications of genetic diversity in pathogen management and the evolutionary dynamics of the Fusarium species causing FSR in maize in the Indian subcontinent.

As environmental and health concerns increase, the trend of sustainable agriculture moves toward using biological agents. About 60% of all biological fungicides have Trichoderma species as the active ingredient, with T. harzianum as the most common species in these products. However, the name T. harzianum has often been incorrectly used in culture collections, databases, and the scientific literature due to the division of the Harzianum Complex Clade (HCC) into more than 95 cryptic species with only one being named T. harzianum. In this study, strains previously identified as T. harzianum in three surveys of Trichoderma species from soils in South and Central America were reanalyzed using phylogeny based on tef1α, rpb2, and ITS loci and combined with the corresponding species in two other studies, where the species were correctly identified. Based on the results of the five surveys, T. afroharzianum, T. lentiforme, T. endophyticum followed by T. azedevio and T. harzianum were found to be the dominant species of the HCC in South and Central America. This is the first report of identifying dominant Trichoderma species within the HCC in South and Central American soil based on multiple studies, which will be useful in selecting strains within the complex clade for formulation of biocontrol and biofertilizer products in the continent.

Certain Trichoderma isolates provide biofertilizer, biocontrol, and other plant-beneficial activities while inhabiting the soil or internal plant tissue, and their use in agricultural systems can contribute to sustainable food production. It is thought that colonization of soil or internal plant tissue is fundamental for biocontrol and biofertilizer applications. Our collective analyses of prior surveys, where the tef1α sequence was almost exclusively used to identify Trichoderma species, showed that isolates from the Harzianum complex clade, the T. asperellum/T. asperelloides group, T. virens, T. hamatum, and T. atroviride were prevalent in soil and/or as endophytes. Population structure and genetic diversity based on the genetic markers tef1α, rpb2, and ITS were investigated, and new lineages with statistical bootstrap support within T. atroviride, T. asperellum, T. hamatum, and T. virens populations were found. The nearest relatives of some of these species were also revealed. Choosing isolates from among more than 500 known Trichoderma species for use in non-targeted evaluation screens for biocontrol or biofertilizer applications is time-consuming and expensive. Preferentially selecting isolates from T. atroviride, T. asperellum/T. asperelloides, T. hamatum, the T. harzianum complex clade, T. virens, and possibly nearest relatives may speed the identification of candidates for commercialization due to the demonstrated ability of these species to successfully inhabit the soil and internal plant tissue. To our knowledge, this is the first report where dominant soil and endophytic Trichoderma species were identified from past survey data and population structure and genetic diversity analyses conducted.