Philosophiae Doctor - PhD / To date five classes of resistance proteins have been identified in plants and these include the intracellular protein kinases, receptor-like protein kinases with extracellular leucinerich repeat (LRR) domain, LRR proteins that encode membrane bound extracellular proteins, toxin reductase and intracellular LRR proteins with a nucleotide-binding site (NBS). These proteins recognise "invading pathogen" and in turn trigger defence response systems that act to protect plants from invading pathogens. The NBS-LRR genes which constitutes the major class encode a family of resistance proteins that are made up of a centrally located nucleotide binding site domain and a C-terminal leucine rich repeat receptor. This class of genes constitute the largest family of resistance genes identified in plants to date. They make up the majority of proteins involved in the plant basal and inducible defence systems against pathogen infection. / South Africa

An evaluation of host-plant resistance to the wireworm-Diabrotica-Systena (WDS) complex of root-feeding insects in sweetpotatoes was performed on a total of 15 cultivars and advanced lines over the course of 4 years (2017-2020). It was found that sweetpotatoes varieties can differ significantly in amount of damage incurred. Several mechanisms of resistance were proposed and tested: periderm toughness, dry weight percentage, and volatile organic chemical defenses. No significant difference was found amongst the varieties tested concerning the physical properties. Chemical analysis was limited in scope but did show some differences between a susceptible variety when compared to a resistant variety in the volatile chemical composition of the foliage. Finally, a commercial kairomone lure with sticky card for Diabroticites was tested for efficacy when compared to sweep net sampling throughout the season. No correlation could be found amongst the methods in terms of number and species of insects caught by each.

Cotton (Gossypium hirsutum L.) crop maturity is delayed by cotton fleahopper (Pseudatomoscelis seriatus Reuter) (fleahopper) feeding on early-season fruit forms which increases vulnerability to late-season pests such as Helicoverpa zea (Boddie) and Heliothis virescens (Fabricius). The objectives of this research were to evaluate methods of screening for resistance to fleahopper and to screen selected genotypes. Six fleahoppers were caged on plants in the insectary for 72 h. Numbers of live fleahoppers and percent square damage were determined 48 h following the removal of fleahoppers. Fleahopper numbers and percent square set were determined on randomly selected plants of 16 genotypes when grown under field conditions in 2002 and 2003. Across multiple sampling dates, the number of fleahoppers per plant was higher (p=0.05) in G. arboreum and Pilose (G. hirsutum), but no consistent differences were observed among the remaining 15 genotypes which represented several germplasm pools across the United States. Field and no-choice feeding tests suggested that Pilose, Lankart 142, Suregrow 747, and Stoneville 474 were more resistant hairy-leaf genotypes and not different (p=0.05) in resistance than the smooth-leaf genotypes, Deltapine 50 and TAM 96WD-69s. Pin-head, match-head, and one-third grown squares were removed from plants and placed on agar in petri-plates. Four fleahoppers were released per plate and allowed to feed for 48 h. Fleahopper damage, brown areas along the anthers and/or brown and shrunken pollen sacs was most evident in pin-head sized squares.

Cotton (Gossypium hirsutum L.) crop maturity is delayed by cotton fleahopper (Pseudatomoscelis seriatus Reuter) (fleahopper) feeding on early-season fruit forms which increases vulnerability to late-season pests such as Helicoverpa zea (Boddie) and Heliothis virescens (Fabricius). The objectives of this research were to evaluate methods of screening for resistance to fleahopper and to screen selected genotypes. Six fleahoppers were caged on plants in the insectary for 72 h. Numbers of live fleahoppers and percent square damage were determined 48 h following the removal of fleahoppers. Fleahopper numbers and percent square set were determined on randomly selected plants of 16 genotypes when grown under field conditions in 2002 and 2003. Across multiple sampling dates, the number of fleahoppers per plant was higher (p=0.05) in G. arboreum and Pilose (G. hirsutum), but no consistent differences were observed among the remaining 15 genotypes which represented several germplasm pools across the United States. Field and no-choice feeding tests suggested that Pilose, Lankart 142, Suregrow 747, and Stoneville 474 were more resistant hairy-leaf genotypes and not different (p=0.05) in resistance than the smooth-leaf genotypes, Deltapine 50 and TAM 96WD-69s. Pin-head, match-head, and one-third grown squares were removed from plants and placed on agar in petri-plates. Four fleahoppers were released per plate and allowed to feed for 48 h. Fleahopper damage, brown areas along the anthers and/or brown and shrunken pollen sacs was most evident in pin-head sized squares.

Whiteflies (Bemisia tabaci, Biotype B, Homoptera: Aleyrodidae) are pests of cotton crops, affecting the yield of the crop both indirectly and directly. These pests feed on the leaves of cotton plants and produce "honeydew," a sticky liquid excretion which covers the lint of the open cotton boll creating problems during the processing of the lint. High densities of these pests also can decrease the productivity of the cotton plant by stripping it of vital nutrients. The primary objective of this research was to screen 116 converted cotton race stocks for resistance to sweetpotato whiteflies. Responses of converted race stocks to whiteflies are compared to that of known susceptible commercial cultivars PSC 355 and Delta Pearl. Screens for antibiosis resistance to whitefly were established using excised leaves placed in a nutrient solution (¼ strength Hoagland's). Cohort populations of whiteflies were established on these leaves and followed daily to determine differences in developmental time as well as percent survival. Resistant candidates were determined using a chi-squared test comparing the ranked sums of leaf averages for the two selection criteria, whitefly developmental time and percent whitefly survival, of each cotton race stock to that of the putative known susceptibles (KS). These tests showed 6 converted race stocks to be significantly different (P ≤ 0.1) from the KS for at least one of the two selection criteria. Of these converted race stocks, M-9044-0154 and M-9044-0156 showed to have lower whitefly survival than the KS, while M-9644-0188, M-9644-0195, M-0044-0221, and M-9644-0242 showed whitefly to have an increased developmental time. Retesting of these six converted race stocks along with several others identified another race stock line, M-0044-0171, which was significantly different from the KS. Retesting also indicated that M-9644-0188 was different for survival in addition to developmental time which was determined in the original screening. Additional tests were conducted in the greenhouse and field to examine these race stock lines. Greenhouse screenings indicated that M-9044-0156 and M-9644-0188 contained possible non-preference resistance characteristics. Field screenings conducted in Weslaco and College Station, TX in 2002 and 2003 failed to provide useful data due to low densities of insects.

Bacterial endosymbionts can have profound impacts on their host’s ecology. Notably, endosymbionts can protect their hosts against natural enemies and influence host plant interactions. The endosymbiont Candidatus Arsenophonus infects a wide taxonomic range of arthropod hosts, and is suspected of an uncharacterized mutualistic role in hemipterous insects. In the soybean aphid, Aphis glycines, an introduced pest of soybeans in the United States, Arsenophonus is the sole facultative endosymbiont. The focus of this dissertation is to characterize the role of Arsenophonus in the aphid, with an overall emphasis on its impact on aphid management strategies. I first used diagnostic PCR to determine Arsenophonus infection frequency and strain diversity for native and introduced soybean aphids. I found that Arsenophonus infection is a uniform strain that is highly prevalent in soybean aphid. I then determined if Arsenophonus was a defense symbiont by curing two genotypes of soybean aphid of their natural Arsenophonus infection, resulting in infected and uninfected isolines within the same genetic background. I subjected these isolines to assays with three parasitoid species and a common aphid fungal pathogen, Pandora neoaphidis. I did not find differences in parasitism or fungal infections within the treatments. These results indicate that, although Arsenophonus is widespread, the symbiont should not interfere with biological control efforts. I next examined the influence of Arsenophonus on the ability of soybean aphid “biotypes” to colonize resistant Rag plants. I cured three additional soybean aphid biotypes. All isolines were subjected to growth rate assays on resistant Rag versus susceptible soybean. My results indicate that Arsenophonus infected soybean aphids have an increased population growth compared to uninfected aphids regardless of soybean plant type Finally, I induced soybean plants with jasmonic acid (JA) or salicylic acid (SA) to determine the effective plant defense against soybean aphid feeding. I also used Arsenophonus infected and uninfected aphids to determine any interaction between Arsenophonus and plant defense. I found SA treatment decreased soybean aphid population growth for one experiment, but had no effect when replicated. JA treatment had no effect, and there were no interactions between Arsenophonus infection and plant treatments.

Two sources of reniform nematode (Rotylenchulus reniformis) resistance in two Upland cotton (Gossypium hirsutum) developed from related cotton species, BARBREN-713 (Renbarb1 and Renbarb2 genes), and the LONREN germplasm lines (Renlon gene) are available to breeders. Stunting was observed in the LONREN source of resistance, but not for BARBREN. To investigate future use of LONREN germplasm, lines were developed with different combinations of genes from both the LONREN and BARBREN-713 germplasm lines. Marker assisted selection (MAS) was conducted identify these gene combinations within breeding population 1) Renbarb1 +Renbabr2 2) Renbarb1 + Renbar2 + Renlon 3) Renbarb2 + Renlon 4) Renbarb2 5) Renbarb1 6) Renbarb1+Renlon 7) Renlon, and 8) a susceptible genotype. Two populations were identified and increased in 2015. Genotypes were inoculated with 0, 2300, or 5000 reniform nematodes. Seedlings were harvested at 90 days to collect plant measurements and RF of reniform nematodes from roots and soil. The Renbarb1 genotype was susceptible. No significant differences in genotypes were observed for root and shoot measurements, and there was no indication of stunting.

No description available.

Strawberry powdery mildew, caused by Podosphaera aphanis, affects leaves, fruit, and runners of strawberry plants. Infected leaves have reduced photosynthetic capability and infected fruit become unmarketable. Both of these factors translate to economic loss for the grower and therefore merit taking measures to control the disease. One objective of this study was to evaluate the resistance developed in populations of strawberry powdery mildew to chemical control measures. A fungicide assay was developed to evaluate the efficacy of six treatments (penthiopyrad, quinoxyfen, myclobutanil, trifloxystrobin, cyflufenamid, fluopyram + trifloxystrobin) for control of the disease. Nineteen isolates of strawberry powdery mildew were collected from Balico, Salinas, Watsonville, San Luis Obispo, Santa Maria, Ventura, and Oxnard CA and tested through the assay. The number of isolates resistant to each treatment was: penthiopyrad (7), quinoxyfen (6), myclobutanil (7), trifloxystrobin (2), cyflufenamid (1), fluopyram + trifloxystrobin (0). This documents resistance in P. aphanis to multiple chemicals used for its control. Documentation of any resistance is novel in California and novel worldwide with resistance to Fungicide Resistance Action Committee (FRAC) codes 7 and 13. Another objective of this study was to evaluate host plant resistance to strawberry powdery mildew. Twelve cultivars were evaluated in a winter greenhouse trial, sixteen cultivars in a summer greenhouse trial, and the ten cultivars shared in both trials were also evaluated in two fields. The cultivars found to be most susceptible to mildew infection were BG 3.324 and Royal Royce. The cultivars found to be the least susceptible to mildew infection were Fronteras, San Andreas, and Sweet Ann. The cultivars evaluated represent more than 55% of the state’s acreage and the host plant resistance information will be a valuable tool to growers looking to culturally control powdery mildew.

Since 2000, stripe rust, caused by the fungal pathogen (Puccinia striiformis Westend. f.sp. tritici Eriks.) has resulted in yield losses of wheat (Triticum aestivium L.) in the United States, that exceeded the combined losses of leaf rust (Puccinia triticina Eriks.) and stem rust (Puccinia gramins Pers.:Pers f. sp. Tritici Eriks. E. Henn.). The objectives of this study are to identify and map adult plant stripe rust resistance quantitative trait loci (QTL) in soft red winter (SRW) wheat that are effective against race PST-100, which is the predominant race of the pathogen in the eastern U.S. Adult plant resistance (APR) was characterized in the resistant wheat lines "USG 3555", VA00W-38, and "Coker 9553". Resistance in each of the lines was evaluated in populations derived from crosses with susceptible lines "Neuse", Pioneer Brand "26R46", and VA01W-21, respectively. On chromosomes 1AS, 4BL, and 7D of USG 3555, QTL were identified that explain on average 12.8, 73.0, and 13.6 percent of the variation for stripe rust infection type (IT), and 13.5, 72.3, and 10.5 percent of the variation for stripe rust severity. A QTL from Neuse was identified on 3A that on average explains 10.9 percent of the variation for IT and 13.0 percent of the variation for severity. On chromosomes 2AS and 4BL of VA00W-38, QTL were identified that on average explain 58.9 and 19.3 percent of the variation for stripe rust IT, and 51.9 and 12.1 percent of the variation for severity. On chromosomes 6BL and 3BL of Pioneer 26R46, QTL were identified that on average explain 8.9 and 2.1 percent of the variation for IT and 11.7 and 3.9 percent of the variation for severity. Coker 9553 possesses the QTL on 4BL that is also present in USG 3555 and VA00W-38. The SSR markers, Xgwm296, Xbarc163, and Xwmc756 were tightly linked to QTL on chromosomes 2AS, 4BL, and 6BL, respectively, and their use and development of additional diagnostic markers will facilitate the incorporation and pyramiding of stripe rust resistance QTL into SRW wheat lines via marker-assisted selection. / Ph. D.