Crop diseases are a constant problem in agricultural production fields across Texas. Several soilborne pathogens of sorghum (Sorghum bicolor) and corn (Zea mays) are endemic problems across Texas but especially in South Texas. Two of the primary soilborne pathogens of sorghum are sorghum downy mildew (Peronosclerospora sorghi) and head smut (Sporosorium reilianum). Long term disease management of these pathogens is accomplished primarily through breeding for host plant disease resistance. The soilborne fungal pathogen Macrophomina phaseolina causes charcoal stalk rot in sorghum and corn and charcoal rot of other dicot crops like soybean and sesame. Most incidence of charcoal rot occurs when established crops are subjected to drought stress environments so drought tolerance can be an important host plant resistance component. On corn, the fungal pathogens Fusarium verticillioides and Aspergillus flavus are associated with the pre-harvest mycotoxins fumonisin and aflatoxin, respectively. The pre-harvest occurrence of these mycotoxins in corn kernels is associated with drought stress environments across most Texas corn production regions. Host plant resistance to mycotoxin accumulation is a primary means for reducing mycotoxin risk in corn. Host plant resistance is our primary disease control strategy but that strategy is also guided by studies of pathogen ecology and survival of plant pathogens, host:parasite:environment interactions, and their contributions to disease development and loss. Where feasible, other disease controls pursued include chemical, cultural, biological, and integrated control approaches.
Reducing Mycotoxin Risk in Corn
Field screening methodologies are ongoing yearly in South Texas to assess aflatoxin (mycotoxin) risk of corn inbreds and hybrids as influenced by biotic (primarily insect) and abiotic (heat, drought) stresses. This research is collaborative with Texas A&M AgriLife corn breeders in Lubbock and College Station to evaluate inbreds and elite hybrids for susceptibility and resistance to aflatoxin accumulation as caused by Aspergillus flavus in South Texas stress environments in the field.
Some corn hybrids subjected to progressively favorable early season environments followed by high heat and water stress during late kernel development can develop a loss of kernel integrity. The splitting of the kernel pericarp and seed aleurone layer at high kernel moistures exposes the endosperm to colonization by various fungi including F. verticillioides and A. flavus which can increase incidence of the their associated mycotoxins. It is critical to conduct field screening in the Texas high plains, South Texas, and perhaps other regions to identify hybrids most vulnerable to loss of kernel integrity.
The progressive field environments associated with loss of kernel integrity are not fully understood and may occur only sporadically at specific locations or regions across Texas. However, these environments occur with enough regularity that there are near yearly localized problems (often unrecognized) with loss of kernel integrity occurring in susceptible hybrids somewhere across many Texas corn production areas. We hope to better determine the primary components of these seasonal progressive stress environments and how they interact with crop developmental stages to produce loss of kernel integrity with associated fungal colonization and mycotoxin accumulation. All of these studies are conducted in collaboration with the Texas A&M AgriLife corn breeder in Lubbock.
Sorghum downy mildew and head smut
Sorghum germplasm is routinely screened for resistance or susceptibility to sorghum downy mildew, head smut, charcoal stalk rot, and other diseases in South Texas sorghum nurseries. Sorghum downy mildew is an endemic disease across the upper and lower coastal bend of Texas. Historically, two variants of pathotype 3 and pathotype 6 of Peronosclerospora sorghi had been an ongoing problem in commercial grain sorghum fields, especially in the upper coastal bend. However, over several years, multiple sources of host plant resistance have reduced producer problems with this disease in commercial sorghum fields.
Beginning in 1980, pathotype 3 overcame resistance genes like those present in TX 430. The pathotype 3 variants are either sensitive or resistant to acylalanine fungicides used as seed treatments. Pathotype 6 (with inherent resistance to acylalanine fungicides) overcame many resistance sources that replaced TX 430. Lines and cultivars with resistance to all known pathotypes of P. sorghi in the U.S. include SC414-12.
Sudangrass cultivars grown commercially or used in production of sorghum-sudan hybrids are typically susceptible to all known pathotypes of P. sorghi. Our project has developed sudangrass cultivars that are resistant to all U.S. pathotypes through use of genes from SC414-12. One sudangrass cultivar from this project has already been publically released through Dr. Bill Rooney’s program (Texas A&M AgriLife Sorghum Breeder, College Station). We are readying several other cultivars for release from my program in the near future.
Johnsongrass as reservoirs for sorghum disease pathogens
We are evaluating feral populations of johnsongrass as survival reservoirs of pathogens and characterizing those populations as potential sources of inoculum that could or do initiate specific diseases in producer grain and forage sorghum fields. This project is a collaborative effort with Dr. Clint Magill of the Department of Plant Pathology, College Station. The genetic diversity across Texas johnsongrass populations includes differential susceptibility to some sorghum pathogens. However, that differential susceptibility may have little or no actual influence on the actual movement of pathogens from johnsongrass to commercial sorghum production fields. A primary exception is sorghum downy mildew caused by Peronosclerospora sorghi.
Our research has shown that the predominant populations of P. sorghi on johnsongrass are acylalanine sensitive, pathotype 3. It may provide initial inoculum for infection of susceptible grain sorghum especially in johnsongrass populations near, bordering, or within commercial fields. Grain sorghums in the upper and lower coastal bend should at least have resistance to pathotype 3 for best protection against sorghum downy mildew. Seed treatments with acylalanine fungicides should be fully effective for a few weeks but, under conducive environments, local lesion spread of P. sorghi may then increase exponentially on hybrids highly vulnerable to pathotype 3. Local lesion spread on susceptible hybrids can be extensive but they typically aren’t directly associated with economic loss. However, some highly vulnerable hybrids may develop large numbers of systemically infected plants that are derived from local lesions. These infected plants have a direct loss of yield, act like weeds, and produce soilborne survival spores (oospores) within the fields often at a great distance from the field perimeter origins.
Acylalanine resistant pathotype 3 and pathotype 6 variants of P. sorghi have no easily detectable survival in feral johnsongrass populations across South Texas. This indicates that acylalanine seed treatment fungicides like metalaxyl and mefenoxam should remain effective even in areas of the upper coastal bend where they had previously lost their efficacy. Alternately, the predominant presence of pathotype 3, acylalanine sensitive populations in nearly all feral johnsongrass populations demonstrates the need for sorghum and forage hybrids to at least have resistance to pathotype 3 of P. sorghi.
Gary Odvody, Associate Professor of Plant Pathology
Jeff Remmers, Research Associate
Yang, C., G. N. Odvody, C. J. Fernandez, J. A. Landivar, R. R. Minzenmayer, and R. L. Nichols. 2015. Evaluating unsupervised and supervised image classsification methods for mapping cotton root rot. Precision Agriculture (Springer Science) 16: 201-215.
Yang, C., G.N. Odvody, J. A. Thomasson, T. Isakeit, and R. L. Nichols. 2016. Change detection of cotton root rot infection over 10-year intervals using airborne multispectral imagery. Computers and Electronics in Agriculture 123 (2016): 154-162.
Yang,C., G. N. Odvody, J. A. Thomasson, T. Isakeit, R. R. Minzenmayer, D. R. Drake, and R. L. Nichols. 2018. Site-specific management of cotton root rot using airborne and high-resolution satellite imagery and variable-rate technology. Transactions of the ASABE Vol. 61(3): 849-858
Ahn, E., L.K. Prom, G. Odvody, and C. Magill. 2018. Responses of johnsongrass against sorghum anthracnose isolates. J. Plant Pathol Microbiol 9: 442. doi:10.4172/2157-7471.1000442
Ahn, E., L.K. Prom, G. Odvody, and C. W. Magill. 2019. Defense responses against the sorghum anthracnose pathogen in leaf blade and midrib tissue of Johnsongrass and sorghum. Physiol. Mol. Plant Pathol. 106:81-86.
Radwan, G. L., L.K. Prom, G. Odvody, and C. W. Magill. 2019. Mating type a locus alleles and genomic polymorphism in Sporisorium reilianum: Comparison of sorghum isolates to those from maize. J. Australasian Plant Pathology, 48(2): 119-129