Location: Insect Control and Cotton Disease Research
2024 Annual Report
Objectives
Objective 1. Develop molecular tools to rapidly identify and detect presence of cotton boll rot pathogens in plants and insects. (NP303, C1, PS1A, PS1B)
Objective 2. Improve the understanding of insect vector/pathogen interactions and pathogen reservoirs to reduce the spread of cotton diseases. (NP303, C2, PS2C, PS2D)
Objective 3. Identify fungal and host genes that can be targeted for disease suppression, and develop cotton germplasm resistant to FOV and nematodes. (NP303, C2, PS2A, PS2B; C3, PS3A, PS3B)
Objective 4. Evaluate national and international sorghum germplasm resources, and determine the inheritance and allelic relationships of host plant resistance to anthracnose, grain mold, head smut, and downy mildew diseases. (NP303, C3, PS3A; C2, PS2A)
Objective 5. Determine the population structure of diverse anthracnose (Colletotrichum sublineolum) and head smut (Sporisorium reilianum) isolates and phenotypic expression patterns of virulence on host differentials. (NP303, C2, PS2A)
Approach
Boll rots, nematodes, and fungal pathogens continue to pose the greatest threat to U.S. cotton production. To address boll rots, we previously sequenced the genome of several bacterial boll rot pathogens vectored by cotton fleahoppers, stink bugs, and verde plant bugs to identify pathogenicity genes involved in boll rot disease. In this project, we will confirm whether Lygus spp. and stink bugs vector boll rot and Fusarium wilt pathogens, respectively, and identify pathogenicity genes common to all bacterial boll rot pathogens with the long-term goal of developing a PCR-based kit that can be used in the field to rapidly detect insects harboring pathogens. The ability to rapidly detect boll rot pathogens within insects and plants, along with an improved understanding of insect vector and pathogen interactions, are both critically needed to develop sound management strategies for cotton diseases and respective insect vectors. A strain of Fusarium oxysporum f. sp. vasinfectum, known as race 4 (FOV4), was recently detected in several cotton fields in West Texas and New Mexico. This strain was initially detected and confined to cotton fields in California. Unlike other FOVs, race 4 does not require the presence of nematodes to cause severe disease of plants; however, the interaction between root knot nematodes and FOV4 pathogenicity remains unclear. Spread of FOV4 in the United States could be catastrophic because there are currently no feasible control options once this pathogen is established in a field. The development of resistant germplasms is widely deemed the most practical and long-term solution for managing this disease. To this end, we will identify and test sources of resistance to FOV4 and develop respective markers which will be incorporated into previously developed nematode-resistant germplasm to facilitate our breeding efforts to produce cotton lines that are resistant to both FOV4 and nematodes.
Progress Report
Work in FY 2024 provided a clearer understanding of the dynamics of two major cotton diseases in the United States, boll rot and Fusarium wilt caused by Fusarium oxysporum f. sp. vasinfectum (Fov). Sorghum research led to the discovery of several lines that are resistant to anthracnose, grain mold, head smut, and downy mildew diseases. In work addressing Objectives 1 and 2, tarnished plant bugs were collected from cotton fields in Texas and in Mississippi that showed signs of boll rot; work was initiated to determine if these insects harbor boll rot pathogens. Research addressing Objective 3 led to the development of several genetically near-identical (isogenic) cotton lines that are resistant or susceptible to the fungal disease, FOV, caused by race 4 (Fov4). The development of these isogenic sister lines that differ in their susceptibility to Fov4 is critical for establishing the underlying mechanism(s) of Fov4 resistance in cotton and might provide potential sources of Fov4 resistance that can be incorporated into breeding programs. Work under Objective 3 also identified several cotton germplasm lines that showed resistance to Fov4 under controlled environmental conditions; the most promising candidates were propagated to increase seed production for field evaluations. In research addressing Objective 4, hundreds of sorghum lines from national and international germplasm resources were screened for resistance to anthracnose, grain mold, head smut, and downy mildew diseases. Based on evaluations in greenhouses, several lines are highly resistant to one or more of these fungal diseases. Some of these resistant lines were crossed with susceptible lines to provide insight on the inheritance patterns of resistance; this information will be critical in developing new disease-resistant sorghum varieties for use by U.S. farmers. Work under Objective 5 led to the collection and identification of eight anthracnose and head smut isolates with varying levels of virulence; the work revealed the variability and associated phenotypic expression patterns of virulence among anthracnose and head smut isolates. Recognition of this variability will be critical in identifying and evaluating sorghum lines that are resistant to these fungal pathogens.
Accomplishments
1. Registration of cotton germplasm lines resistant to both root-knot and reniform nematodes. Among cotton pests, plant-parasitic nematodes (e.g., reniform and root-knot nematodes) are responsible for high yield losses. Over the past 20 years, U.S. cotton farmers have experienced losses to the reniform and root-knot nematodes of about $250 million annually, more than all other cotton diseases combined. These nematodes can also interact synergistically with root rot pathogens such as Fusarium oxysporum f. sp. vasinfectum (Fov) to exacerbate disease severity. Although repeated applications of pesticides, along with crop rotation, can reduce nematode damage, the development and use of nematode-resistant cultivars is widely considered to be the most effective, economical, and sustainable approach for managing nematodes in cotton. ARS researchers at College Station, Texas, in collaboration with Cotton Incorporated and academic colleagues, developed and registered three germplasm lines with greater yields, and increased fiber length and strength, as compared to several elite commercial cultivars. These lines are stacked with two root-knot resistant genes and two reniform resistant genes all in homozygous states. The lines also exhibited significant reduction in Fusarium wilt severity. The registration and availability of these lines is already facilitating work by breeders to develop new commercial varieties that are resistant to nematodes and that will concurrently provide resistance against strains of FOV that require the presence of nematodes for infection.
2. New pathotypes of the anthracnose pathogen, Colletotrichum sublineola. The sorghum anthracnose pathogen causes significant annual economic losses estimated to be in the hundreds of millions of dollars worldwide. Management of this disease can be challenging because of the variability within the pathogenic population. Thus, knowledge of the pathotypes that exist within sorghum production regions will be critical in the precise pyramiding of host plant resistance genes to develop effective anthracnose resistant sources. ARS researchers at College Station, Texas, and Mayaguez, Puerto Rico, in collaboration with academic colleagues, determined the virulence pattern of several pathotypes from Georgia, North Carolina, Puerto Rico, and Texas, and documented new pathotypes of the anthracnose pathogen. This accomplishment is significant because it provides new knowledge of the pathotypes that exist in different growing regions. Sorghum breeders and researchers will exploit this knowledge in either targeted development of resistant sorghum lines, or identification of commercial lines that can be more widely used in the United States and abroad to minimize anthracnose damage.
Review Publications
Prom, L.K., Cuevas, H.E. 2023. Reaction of sorghum differentials to grain mold infection in Puerto Rico. American Journal of Plant Sciences. 14:1207-1213. https://doi.org/10.4236/ajps.2023.1411081.
Prom, L.K., Botkin, J.R., Ahn, E.J., Sarr, M., Diatta, C., Fall, C., Magill, C.W. 2023. A genome-wide association study of Nigerien and Senegalese sorghum germplasms of Exserohilum turcicum, the causal agent of leaf blight. Plants. 12. Article 4010. https://doi.org/10.3390/plants12234010.
Prom, L.K., Cuevas, H.E., Ahn, E.J., Isakeit, T.S., Magill, C.W. 2024. Correlations among agronomic traits obtained from sorghum accessions planted in a field infected with three important fungal diseases. Journal of Plant Studies. 13(1). Article 11. https://doi.org/10.5539/jps.v13n1p11.
Prom, L.K., Ahn, E.J., Perumal, R., Cuevas, H.E., Rooney, W.L., Isakeit, T.S., Magill, C.W. 2023. Genetic diversity and classification of Colletotrichum sublineola pathotypes using a standard set of sorghum differentials. The Journal of Fungi. 10(1). Article 3. https://doi.org/10.3390/jof10010003.
Prom, L.K., Sarr, M.P., Diatta, C., Sall, M., Bodian, S., Fall, C., Dorego, G. S. and Magill, C. 2023. A survey of the major sorghum production regions for foliar and panicle diseases during the 2022 growing season in Senegal, West Africa. American Journal of Plant Sciences. 14:829-844. https://doi.org/10.4236/ajps.2023.148055.
Cuevas, H.E., Knoll, J.E., Prom, L.K., Stutts, L.R., Vermerris, W. 2023. Genetic diversity, population structure and anthracnose resistance response in a novel sweet sorghum diversity panel. Frontiers in Plant Science. 14. Article 1249555. https://doi.org/10.3389/fpls.2023.1249555.
Medrano, E.G., Barlow, V., Liu, J., Prom, L.K. 2023. Brown stink bug, Euschistus servus (Say) damage cotton Gossypium hirsutum (L.) in California's southern desert fields. Research Journal of Plant Pathology. 6(3). Article 161. https://doi.org/10.36648/iprjpp.6.2.161.
Cuevas, H.E., Prom, L.K. 2024. Association analysis of grain mould resistance in a core collection of NPGS Ethiopian sorghum germplasm. Plant Genetic Resources. https://doi.org/10.1017/S1479262124000157.
Prom, L.K., Ahn, E.J., Perumal, R., Isakeit, T.S., Odvody, G.N., Magill, C.W. 2024. Genetic and pathogenic variability among isolates of Sporisorium reilianum causing sorghum head smut. The Journal of Fungi. 10(1). Article 62. https://doi.org/10.3390/jof10010062.