<p> Tan spot and Septoria nodorum blotch cause serious yield losses in winter wheat in North Dakota as the majority of commercially grown cultivars are susceptible. This study aimed to identify lines with improved resistance for use as breeding parents. First, advanced NDSU breeding lines and alternative sources of resistance were inoculated with fungal isolates and tested for necrotrophic effector sensitivity. Second, resistant lines were derived from a highly heterogeneous recurrent mass selection F₂ population using single seed descent inbreeding coupled with selection for resistance. Finally, the best performers from both experiments (total of 52 lines) were evaluated to confirm resistance. In addition the 52 lines were analyzed with markers that detect <i>Tsn1</i> and the 1RS rye translocation. Twenty lines were identified with simultaneous resistance to four or three fungal isolates and insensitivity to three, two, or one necrotrophic effectors (of which 11 can be used directly as new parents).</p>

The sooty blotch and flyspeck (SBFS) complex causes blemishes on apples in humid, temperate growing regions worldwide. In contrast to flyspeck etiology, the many species of fungi causing sooty blotch (SB) have not been well studied. The first set of objectives in this study was to use PCR to identify SB species isolated from apples and selected reservoir hosts in the northeastern United States, and to identify patterns of species distribution on hosts and among sites. Results indicated that Geastrumia polystigmatis was the predominant species on apples, whereas Peltaster species were more common on reservoir hosts. Species distribution varied among sites. Phylogenetic analysis of 54 G. polystigmatis isolates revealed little genetic variability in the ITS region. The second set of objectives involved investigating the response of G. polystigmatis to changes in nutrition, temperature, heat stress, and relative humidity, and in vitro responses of G. polystigmatis and Peltaster fructicola to fungicides commonly used in orchards. Observation of growth on half-strength potato dextrose agar, malt extract agar, and 2% water agar revealed that mycelial growth of G. polystigmatis was thicker and more melanized in the presence of readily available carbohydrates. Temperature range experiments demonstrated that the optimum temperature for growth was approximately 24ºC. The fungus was able to survive exposure to 32ºC for at least one week, 37ºC for at least 48 hours, and 42ºC for at least 8 hours. Growth was optimum at 99-100% relative humidity. Isolates of P. fructicola were very sensitive to thiophanate-methyl, mancozeb, cyprodinil, penthiopyrad, fenbuconazole, and trifloxystrobin. Isolates of G. polystigmatis were sensitive to thiophanate-methyl and cyprodinil, but significantly less sensitive to all other fungicides than P. fructicola. The addition of salicylhydroxamic acid to trifloxystrobin significantly reduced growth of P. fructicola, but not that of G. polystigmatis. This study represents the first in-depth investigation into the identity of species causing SB in the Northeast, the basic biology of G. polystigmatis, and the fungicide sensitivities of G. polystigmatis and P. fructicola.

<p> California grows more than 91% of fresh strawberries in the United States. Critical to this success has been management of soilborne diseases using pre-plant soil fumigation with methyl bromide. However, international regulations require a phase out of methyl bromide, soon to be completed. Reduced availability of methyl bromide has coincided with increased incidence of soilborne diseases affecting strawberry production, including Verticillium wilt, caused by <i> Verticillium dahliae</i>, and Black Root Rot (BRR). BRR is caused by a complex of soilborne pathogens, including <i>Pythium ultimum</i>, that form lesions on tertiary roots, which are critical to nutrient and water uptake. Consequently, non-chemical alternatives for sustainable management of soilborne diseases and highly productive plants are urgently needed. </p><p> Crop rotation with legumes can contribute to plant productivity and disease management by fixing nitrogen and providing a non-host interval during which the pathogen can die by natural attrition. However, rotation crops that appear to be non-hosts because they show no symptoms of disease may nevertheless support development of the pathogen and thus negate the benefit of crop rotation. One objective of this research was to evaluate systemic colonization of ten legume cover crops <i>V. dahliae</i> under field conditions and the extent to which plant residue supports development of <i>V. dahliae </i> microsclerotia (Chapter 1). This included seven cool season legumes: broad 'Windsor' bean, bell bean, field pea, hairy vetch, common vetch, purple vetch and 'Lana' woolypod vetch, and three warm season legumes: sesbania, sunn hemp and black-eyed pea. Frequency of systemic infection at ten weeks ranged from 5% (woolypod vetch) to 23% (field pea) and at the end of the trial ranged from 0% (purple vetch) to 23% (hairy vetch). The trend for mean density of microsclerotia in residue at ten weeks ranged from 0 CFU/g residue (hairy vetch) to 583 CFU/g residue (field pea) and at the end of the trial ranged from 63 CFU/g residue (broad bean) to 1096 CFU/g residue (field pea). In most cases, frequency of infection and formation of microsclerotia in plant residue was higher by the end of the trial than at ten weeks. Thus, in fields infested with <i>V. dahliae</i>, growers should avoid rotation with the evaluated legumes to avoid increasing soil inoculum levels. </p><p> Compost can contribute to plant productivity and disease management by improving soil structure and fertility, and providing the necessary factors to shift soils from disease conducive to suppressive. This study was undertaken to evaluate four composts that are available to California strawberry growers: manure compost, spent mushroom compost, vermicompost and yard trimmings compost. The objective was to evaluate the effect on production parameters including soil quality and fertility, and plant growth and yield. Manure and mushroom compost significantly increased soil electrical conductivity, which reached levels of 9.9&plusmn;1.7dS/m and 7.3&plusmn;0.8dS/m, respectively. Manure, yard trimmings and mushroom composts shifted soil pH closer to optimal levels for up to 7 months in 4 to 5 of the trials. Mushroom compost had the greatest effect on soil nitrate, with up to 32 mg/kg nitrate higher than the non-amended soil. </p><p> Another potential benefit of compost is suppression of soilborne pathogens, which can result from changes in the composition and activity of the soil microbiota. It was an objective of this study to determine if four commercially available composts influence infection of strawberry roots by <i>V. dahliae </i> and <i>P. ultimum.</i> The results showed a significant reduction in <i>V. dahliae</i> root infections in some compost amended soils but results were not consistent across trials. The effect of compost amendments on seedling disease caused by <i>P. ultimum</i> was a reduction in disease incidence by 38-43% compared to the non-amended soil. </p><p> The industry-wide shift in strawberry production generates a tremendous need for knowledge transfer and grower support. Accordingly an additional objective of this research was to solicit industry perspectives on the status of soilborne disease management. Results identified crop rotation as the most important tool in the absence of fumigation as, reported by 46% of respondents. When given a choice of thirteen management tools, crop rotation also had the highest ranking by respondents as a practice always used/recommended. </p><p> Numerous soilborne disease management tools, like crop rotation and compost, sustain high yields and reduce disease incidence, but vary in status of adoption. At a pivotal time when land is still productive but pathogens are becoming more widespread, a regional plan for maintaining pathogen-free soil has an opportunity to emerge as the foundation for a sustainable industry in the post-fumigation era. (Abstract shortened by UMI.)</p>

<p> <i>Aspergillus flavus</i> infection of agriculturally important crops such as tree nuts, maize, peanuts, and cotton has decreased crop value. Researchers have identified three major approaches to combat <i>A. flavus </i> growth and aflatoxin accumulation: identifying natural resistance in crops, genetically engineering crops for enhanced resistance, and introducing an atoxigenic fungal strain as a competitor. In this dissertation, I investigated two of the three means to control <i>A. flavus</i> growth and infection: genetically engineered crops and identification of natural resistance. My studies of natural resistance in cotton crop show that Sa 1595, a <i> Gossypium hirsutum</i> cultivar, is significantly more susceptible to <i> A. flavus</i> infection; however, no significantly resistant cultivars were observed, but I did observe a trend of diminished susceptibility in A2 186 and Tamcot Sp 23. I then examined synthetic antimicrobial peptide, D4E1, as a means to increase resistance in crops. My research shows that D4E1 effectively increases reactive oxygen species (ROS), an apoptosis precursor at concentrations as low as 1 &micro;M. Breaches in the membrane that allow infiltration and subsequent fluorescence from Sytox^&reg; green occur at higher concentrations. Finally, genetically engineered tobacco plants were examined for D4E1 localization. My research shows that the HA-D4E1 construct was present in the most abundance in the chloroplast of plastid transformed plants, while nuclear transformed plants had nuclear localization. All of my findings suggest that cotton crops do not exhibit any significant enhanced natural resistance to <i>A. flavus</i> infection and growth; however, engineering crops with D4E1 will exhibit enhanced crop resistance.</p><p>