Drought stress decreases crop growth, yield, and can further exacerbate pre-harvest aflatoxin contamination. Tolerance and adaptation to drought stress is an important trait of agricultural crops like maize. However, maize genotypes with contrasting drought tolerances have been shown to possess both common and genotype-specific adaptations to cope with drought stress. In this research, the physiological and metabolic response patterns in the leaves of maize seedlings subjected to drought stress were investigated using six maize genotypes including: A638, B73, Grace-E5, Lo964, Lo1016, and Va35. During drought treatments, drought-sensitive maize seedlings displayed more severe symptoms such as chlorosis and wilting, exhibited significant decreases in photosynthetic parameters, and accumulated significantly more reactive oxygen species (ROS) and reactive nitrogen species (RNS) than tolerant genotypes. Sensitive genotypes also showed rapid increases in enzyme activities involved in ROS and RNS metabolism. However, the measured antioxidant enzyme activities were higher in the tolerant genotypes than in the sensitive genotypes in which increased rapidly following drought stress. The results suggest that drought stress causes differential responses to oxidative and nitrosative stress in maize genotypes with tolerant genotypes with slower reaction and less ROS and RNS production than sensitive ones. These differential patterns may be utilized as potential biological markers for use in marker assisted breeding.

Phytophthora capsici, the causal agent of Phytophthora blight, is a major concern in vegetable production in Georgia and many other states in the United States. Contamination of irrigation water sources by P. capsici may be an important source of inoculum for the pathogen. A simple method was developed in this study to improve the efficiency of recovering P. capsici from fruits used as baits in irrigation ponds. In contrast to direct isolation on agar plates, infected fruit tissues were used to inoculate stems of pepper seedlings, and the infected pepper stems were used for isolation on agar plates. With isolation through inoculation of pepper stems, the frequency of recovering P. capsici from infected eggplant and pear fruits increased from 13.9% to 77.7% and 8.1% to 53.5%, respectively, compared with direct isolation on agar plates. P. capsici was isolated from seven out of nine irrigation ponds evaluated, with most of the ponds containing both A1 and A2 mating types and a 4: 5 ratio of A1 to A2 when isolates from all ponds were calculated. All P. capsici isolates were pathogenic on squash plants, and only a small proportion (8.2%) of the isolates were resistant or intermediately sensitive to mefenoxam. Simple sequence repeats (SSRs) were identified through bioinformatics mining of 55,848 publicly available expressed sequence tags of P. capsici in dbEST GenBank. Thirty-one pairs of SSR primers were designed, and SSR analysis indicated that the 61 P. capsici isolates from irrigation ponds were genetically distinct. Cluster analysis separated the isolates into five genetic clusters with no more than two genetic groups in one pond, indicating relatively low P. capsici genetic diversity in each pond. The isolation method and SSR markers developed for P. capsici in this study could contribute to a more comprehensive understanding of the genetic diversity of this important pathogen.

A loop-mediated isothermal amplification (LAMP) assay was developed for simple, rapid and efficient detection of Cucurbit leaf crumple virus (CuLCrV), one of the most important begomoviruses that infects cucurbits worldwide. A set of six specific primers targeting a total 240 nt sequence regions in the DNA A of CuLCrV were designed and synthesized for detection of CuLCrV from infected leaf tissues using real-time LAMP amplification with the Genie^® III system, which was further confirmed by gel electrophoresis and SYBR™ Green I DNA staining for visual observation. The optimum reaction temperature and time were determined, and no cross-reactivity was seen with other begomoviruses. The LAMP assay could amplify CuLCrV from a mixed virus assay. The sensitivity assay demonstrated that the LAMP reaction was more sensitive than conventional PCR, but less sensitive than qPCR. However, it was simpler and faster than the other assays evaluated. The LAMP assay also amplified CuLCrV-infected symptomatic and asymptomatic samples more efficiently than PCR. Successful LAMP amplification was observed in mixed virus-infected field samples. This simple, rapid, and sensitive method has the capacity to detect CuLCrV in samples collected in the field and is therefore suitable for early detection of the disease to reduce the risk of epidemics.

Fusarium wilt of watermelon, caused by Fusarium oxysporum f. sp. niveum (FON), is a devastating soilborne disease in watermelon production. Race 1 and race 2 of the pathogen are widely distributed in different watermelon producing regions. To investigate whether the root-knot nematodes break resistance of watermelon genotypes with resistance to FON race 1 and race 2, four greenhouse experiments were conducted on two watermelon cultivars ‘Calhoun Gray’ and ‘Fascination’ that are resistant to race 1, and an accession PI 296341-FR that is resistant to race 2. The treatments included seedlings of ‘Calhoun Gray’ and ‘Fascination’ inoculated with Meloidogyne incognita (1000 J2/plant) or FON race 1 alone, nematodes 5 days before FON, FON 5 days before nematodes, nematodes and FON simultaneously, and non-inoculated control. PI 296341-FR was inoculated with FON race 2, with or without M. incognita inoculation. The presence of M. incognita enhanced the susceptibility of all watermelon genotypes to Fusarium wilt. Co-inoculation with M. incognita led to an early development of wilt symptoms and increased disease severity. Galls were observed on roots of all nematode-inoculated plants, and sequential inoculation of FON followed by nematodes resulted in numerically lower galling indices compared to other inoculation methods. Whereas inoculation of the nematodes alone did not reduce plant growth, growth suppression was evident when seedlings were inoculated with both pathogens. The results indicated that M. incognita could enhance susceptibility of resistant watermelon genotypes to respective FON races, and host resistance alone is not sufficient for managing Fusarium wilt on watermelon in soils infested with root-knot nematodes.

Aims The objective of this study was to determine the effects of sophorolipids on several fungal and oomycete plant pathogens and the relationship between sophorolipids at different pH and antimicrobial activities. Methods and Results Sophorolipids had different solubility at different pH with a dramatic increase in solubility when pH was 6 or higher. Inhibition of mycelial growth of Phytophthora infestans by sophorolipids was affected by pH values, showing that when the pH value was higher, the inhibition rate was lower. Sophorolipids inhibited spore germination and mycelial growth of several fungal and oomycete pathogens in vitro including Fusarium sp., F. oxysporum, F. concentricum, Pythium ultimum, Pyricularia oryzae, Rhizoctorzia solani, Alternaria kikuchiana, Gaeumannomyces graminis var. tritici and P. infestans and caused morphological changes in hyphae by microscope observation. Sophorolipids reduced β‐1,3‐glucanase activity in mycelia of P. infestans. In greenhouse studies, foliar application of sophorolipids at 3 mg ml⁻¹ reduced severity of late blight of potato caused by P. infestans significantly. Conclusion Sophorolipids influenced spore germination and hyphal tip growth of several plant pathogens and pH solubility of sophorolipids had an effect on their efficacy. Application of sophorolipids reduced late blight disease on potato under greenhouse conditions. Significance and Impact of the Study The findings indicated that sophorolipids have the potential to be developed as a convenient and easy‐to‐use formulation for managing plant diseases.