Seed Pathology

Seed pathology research encompasses the study of diseases affecting seed production and utilization, as well as disease management practices applied to seeds. The Seed Pathology research program, implemented by resident pathologists Silvina Arias, Charles Block, Derrick Mayfield, and Gary Munkvold, emphasizes three areas: (1) etiology and management of multiple pathogens and pests attacking seeds and seedlings, (2) the management and impacts of mycotoxigenic fungi in maize, and (3) seed health test development.

Etiology and management of pathogens and pests attacking seeds and seedlings

Seeds must perform in a soil environment rich with microbial populations, including multiple pests and pathogens (e.g., fungi, viruses, nematodes, and insects) acting together. Studies in this area strive to better understand how these pathogens interact; they also examine how they can be managed. Some studies in this area:

Seedling pathogens in the soybean production cycle: Management and communication. SSC pathologists are part of a national research team aiming to help farmers reduce yield loss from an array of soybean seedling diseases. Studies have uncovered a number of pathogens that combine to create a disease complex. Management options, therefore, should consider the organisms involved. Part of this multi-state research project explores how seed quality influences seedling diseases caused by soilborne pathogens. The effects of soil physical properties on Fusarium infection were determined, and the baseline sensitivity of Fusarium isolates to seed treatment fungicides was evaluated. Center pathologists are also evaluating the relationship between iron deficiency chlorosis and Fusarium diseases in soybeans and are examining the interactions between susceptibility to seedling and root rot pathogens and tolerance to iron deficiency chlorosis, as well as the role of fungal siderophores as a virulence factor of Fusarium graminearum in iron-deficient environment. Substantial improvements have been made in developing an effective protocol for greenhouse screening of iron deficiency chlorosis and Fusarium root rot using a hydroponics system. This progress paved the way for collaboration with other Iowa State researchers. This study uses a nutrient solution that mimics more closely Iowa’s soils with iron deficiency chlorosis (IDC).

Root infection of soybeans by Fusarium species: Seed and seedling disease, genotypic and genotypic and phenotypic diversity in Fusarium oxysporum from soybean.  In this study, researchers did a phylogenetic analysis of F. oxysporum from soybeans and gene expression in soybeans in response to infection. They also investigated the putative fungal effector proteins that correspond with the pathogenicity of F. oxysporum to soybean or common bean. The pathogenicity of a range of Fusarium oxysporum strains was evaluated across diverse soybean germplasm, and later analysis sought to identify genetic markers related to resistance to Fusarium root rot in soybean.

  

Application of innovative intercropping practices to increase soybean production in Iowa and managing soybean diseases using relay cropping with small grains and oilseeds. In this project, we propose expanding soybean production and potentially increasing the economic returns to Iowa farmers and industry leaders by a sustainable intensification of the existing cropland. Our strategy is to apply intercropping practices, which involve the harvest of our main cash crop- soybean- plus the harvest of a second crop (small grains/oilseeds) in the same field, in the same year, reducing negative environmental costs. One of the main objectives of this project is to evaluate the potential impact of winter small grains/oilseeds in reducing soybean diseases and soybean cyst nematodes. Our purpose is to support farmers in dealing with site-specific disease management and the challenges of managing polycultures.

Role of seed transmission in etiology of vegetable diseases. In this project, the research team developed risk assessment models for seed-transmitted pathogens with tomato canker, Cucumber green mottle mosaic virus, and gummy stem blight (caused by Stagonosporopsis spp.) as model systems. A model that incorporates quality management practices in risk assessment was developed to be tested in real-life seed production scenarios. Risk estimates were generated and their sensitivity to changes in management practices were ascertained. Experimental evaluation of seed transmission of Stagonosporopsis spp. was carried out on diverse Cucurbit crop varieties, and the ability of seed treatment fungicides to reduce seed transmission was established.

Gene expression characterization for understanding combined abiotic and biotic stresses: Iron deficiency and soilborne pathogens interaction in soybean seedlings. In this project, transcriptional changes in soybean roots that have been infected by pathogens growing in an environment with low iron availability will be studied. The findings generated will be useful for determining environmental conditions and stress factors on the epidemiology of soilborne pathogens that affect seedlings and taking steps toward identifying effective management. In addition, these results may be useful in developing new methods of broadening the resistance of soybeans to combined stresses.

Role of seeds in the epidemiology of Xanthomonas vasicola (bacterial leaf streak) in maize. Center scientists developed methods to detect X. vasicola in seeds and investigated the role of seed transmission for this disease, first reported in the U.S. in 2016. Initial assessment of seed lots from diseased fields detected the bacterium at low levels in a small percentage of samples. The study demonstrated seed transmission with inoculated seed, but results indicated that naturally contaminated samples had a negligible risk of seed transmission. These results helped to avert unnecessary phytosanitary restrictions.

Epidemiology and management of seed-borne viruses causing maize lethal necrosis (MLN) in Eastern Africa. In this project, Center scientists investigate the occurrence of internal and external virus contamination of maize seeds from Kenya and the U.S. They are also determining the relationship between the type of seed contamination and the risk of seed transmission and are testing various treatments to manage seed-borne MCMV to prevent outbreaks of MLN in the field.

Effect of winter cereal rye cultivar selection and herbicide choice on seedling disease, pathogen populations, nitrogen dynamics, and growth and yield of corn. In this project, researchers of the Center specifically contribute to the analysis of allelochemicals when rye is used as a cover crop in a corn-soybean production system. The findings of this study will help to understand the biological phenomena of rye allelopathy and their effect on corn growth and soilborne pathogens.

Corn processing treatments to eliminate phytosanitary risk. Trials were conducted to assess microwave treatments for eliminating fungal contamination of maize grain for export.

Nanoparticle encapsulation to enhance seed treatment efficacy. SSC researchers collaborated with scientists in Chemical Engineering to assess the benefits of encapsulating seed treatment fungicides with nanomaterials to prolong seedling protection from fungal pathogens.

Effects of seed treatments on soybean microbiome. This project seeks to elucidate the temporal impacts of chemical and biological seed treatments on pathogenic and beneficial organisms in the soybean rhizosphere and endosphere.

Seedling bioassay for Sphacelotheca reiliana in corn. The SSC pathology team established a method to assess seedling infection by this fungus using real-time PCR. The method has been used to successfully demonstrate the efficacy of several seed treatment fungicides for head smut management.

Mycotoxigenic fungi in maize: Management and impacts

Mycotoxin-producing fungi are often seed-borne or attack plants at the seedling stage. These fungi can have important impacts on seed production and utilization in maize.

Role of mycotoxins in maize seedling diseases caused by Fusarium species. Mycotoxins are harmful to livestock and people, but they can play a role in how plant diseases develop. Studies in this area have investigated the role of fumonisins and deoxynivalenol in the development of diseases in corn, the development of mycotoxin non-producing mutants, and the development of qPCR assessment of maize seedling colonization by wild-type and DON non-producing isolates of F. graminearum and F. verticillioides.

Insect interactions with mycotoxin-producing fungi in stored maize. Scientists investigated the role of Coleopteran and Lepidopteran insects in the contamination of stored maize by aflatoxins and fumonisins, and the value of transgenic insect protection to reduce the risk of mycotoxin contamination in storage.

Role of Aspergillus species in fumonisin contamination of maize grain and impacts on maize seed germination. Aspergillus flavus is an important mycotoxin-producing fungus in corn, but there are other Aspergillus species that infect corn and produce other mycotoxins, including fumonisins. This study investigated the diversity of these species and tested their role in mycotoxin contamination of corn, their effects on the germination of corn seeds and on seedling growth.

Importance of Fusarium temperatum as an ear rot pathogen and mycotoxin-producing species in maize. F. temperatum is a recently described species and SSC research evaluated the role of this fungus in corn seed infection and mycotoxins contamination.

Seed health test development

The development and standardization of seed health testing is critical for international trade in seeds. SSC administration of the U.S. National Seed Health System (NSHS) includes identifying and addressing seed health test development needs to improve safe international seed trade and promote science-based phytosanitary regulations.

Seed health test development for Fusarium oxysporum f. sp. phaseoli on Phaseolus beans. In collaboration with the Federal University of Lavras in Brazil, Center pathologists developed a quantitative DNA-based seed health test for this important pathogen.

Potato spindle tuber viroid and other viroids. These pathogens are seed-transmitted in tomatoes and other crops. SSC participated in optimization of methods for their detection and studied the seed transmission of PSTVd and TASVd in order to support risk assessments.

A PCR method of detecting High Plains wheat mosaic virus in seed corn. High Plains wheat mosaic virus, widespread throughout the southwestern Great Plains states, became a worldwide phytosanitary export concern in the second half of 2022. Chile’s Servicio Agrícola y Ganadero (SAG) issued an emergency rule in mid-November 2022 that required corn seed from the U.S. to be certified free from High Plains virus through PCR testing. SSC scientists are working with counterparts in Bayer Crop Science to create an NSHS-approved lab assay.

Development and validation of a real-time PCR method for the detection of Pantoea stewartii subsp. stewartii in maize seeds. This project aimed to optimize, extend the applicability, and conduct an inter-laboratory validation of the developed DNA test method to verify seed performance criteria according to NSHS validation guidelines. Improved testing methods reduce seed import rejections or crop destruction associated with false-positives known to occur with available testing methods. The intra-laboratory validation study used seed extract SYBR-PCR to detect the bacterium. Researchers also organized and characterized infested seed lots for the comparative tests.

Harmonization of seed health testing method for Pseudomonas syringae pv. glycinea in soybean seeds. This is a validation study to harmonize the NSHS and ISTA methods for detecting this pathogen in soybean seeds. It is a pilot project to develop a process to generate validation data to support simultaneous approval of seed health testing methods by both NSHS and ISTA.

Vegetable and field crop seed health test development. Efforts in this area include: The development of a loop-mediated isothermal amplification (LAMP) for late wilt in maize seeds; harmonization of pepper mild mottle virus and tomato brown rugose fruit virus testing methods between the U.S. and Mexico and optimization of seed extraction methods for maize chlorotic mottle virus