@article{kikway_keinath_ojiambo_2023, title={Within-Season Shift in Fungicide Sensitivity Profiles of Pseudoperonospora cubensis Populations in Response to Chemical Control}, volume={4}, ISSN={["1943-7692"]}, DOI={10.1094/PDIS-09-22-2056-RE}, abstractNote={Cucurbit downy mildew, caused by Pseudoperonospora cubensis, is an important disease affecting cucurbits worldwide. Chemical control is the most effective method for disease control but P. cubensis has a high risk for developing resistance to fungicides. Alternating fungicides with different modes of action is recommended to avoid an increase of resistant subpopulations. Thus, this study was conducted to establish shifts in the sensitivity profiles of P. cubensis isolates during the growing season wherein chlorothalonil was applied in alternation with either cymoxanil, fluopicolide or propamocarb in field experiments conducted from 2018 to 2020 at Rocky Mount, North Carolina and in 2018 and 2020 at Charleston, South Carolina. The sensitivity of baseline isolates sampled early (i.e., baseline isolates) or exposed isolates sampled late in the season, to these single-site fungicides was determined using the detached leaf assay where tested isolates were classified as sensitive or resistant based on the relative disease severity. Based on the Kruskal-Wallis test, the distribution profile of relative mean disease severity among baseline and exposed isolates was significantly different where chlorothalonil was alternated with fluopicolide (P = 0.001) but not with cymoxanil (P = 0.238) or propamocarb (P = 0.802). Although there was a directional selection towards resistance for isolates sampled from plots that were treated with fluopicolide or propamocarb alternated with chlorothalonil during a growing season, a significant shift in fungicide sensitivity distribution based on combined data were observed for fluopicolide (ꭓ2 = 8.25; P = 0.004) but not propamocarb (ꭓ2 = 1.05; P = 0.461). Early- and late-season P. cubensis isolates sampled from the cymoxanil treated plots were all resistant to this fungicide and there was no significant shifts in their fungicide sensitivity profile during a growing season (ꭓ2 = 0.06; P = 1.000). This results indicate that a shift towards reduced sensitivity in P. cubensis can occur during a growing season and the efficacy of fluopicolide is likely to decrease as the frequency of the less sensitive subpopulations increases during a production season. The resultant effect on disease severity and selection of an insensitive subpopulation may accelerate the development of resistance to propamocarb in the southeastern United States.}, journal={PLANT DISEASE}, author={Kikway, Isaack and Keinath, Anthony P. and Ojiambo, Peter S.}, year={2023}, month={Apr} }
 @article{kikway_keinath_ojiambo_2022, title={Field Occurrence and Overwintering of Oospores of Pseudoperonospora cubensis in the Southeastern United States}, volume={8}, ISSN={["1943-7684"]}, DOI={10.1094/PHYTO-11-21-0467-R}, abstractNote={In the United States, the cucurbit downy mildew pathogen, Pseudoperonospora cubensis, has been shown to form oospores under laboratory conditions, but there are no reports on the formation of oospores under natural field conditions. This study investigated the occurrence of oospores in fields in North Carolina and South Carolina from 2018 to 2020. Oospore viability and survival was also determined outdoors during the winter in North Carolina during this period using soil containing leaves infested with oospores. About 5% of 1,658 infected leaves sampled during the study had oospores, with a mean density of 585 oospores per cm2 of infected leaf tissue. Oospore viability as assessed using the plasmolysis method, declined steadily during the exposure period from 67.8% in November to 19.3% in May. Other variables being equal, the decrease in oospore viability was significantly affected by soil temperature (b = -0.03 to -0.05; P < 0.0001) and number of rainy days (b = 21.6 to 40.46; P < 0.05), while the effects of soil moisture on viability were less clear. About 20% of the exposed oospores were viable at the end of the study period. However, these oospores failed to germinate or initiate disease when inoculated onto cucurbit leaves. Thus, oospores might still require some unrecognized stimuli to initiate germination and infection. Nonetheless, viability at the end of the winter period suggests that once exposed to the right conditions, oospores could potentially serve as a primary inoculum source in locations that experience cold winters in the southeastern United States.}, journal={PHYTOPATHOLOGY}, author={Kikway, Isaack and Keinath, Anthony P. and Ojiambo, Peter S.}, year={2022}, month={Aug} }
 @article{kikway_keinath_ojiambo_2022, title={Temporal Dynamics and Severity of Cucurbit Downy Mildew Epidemics as Affected by Chemical Control and Cucurbit Host Type}, volume={106}, ISSN={["1943-7692"]}, DOI={10.1094/PDIS-09-21-1992-RE}, abstractNote={Cucurbit downy mildew caused by the oomycete Pseudoperonospora cubensis is an important disease that affects members of Cucurbitaceae family globally. However, temporal dynamics of the disease have not been characterized at the field scale to understand how control strategies influence disease epidemics. Disease severity was assessed visually on cucumber and summer squash treated with weekly alternation of chlorothalonil with either cymoxanil, fluopicolide or propamocarb, during the 2018 spring season and 2019 and 2020 fall seasons in North Carolina, and the 2018 and 2020 fall seasons in South Carolina. Disease onset was observed around mid-June during the spring season and early September during the fall season, followed by a rapid increase in severity until mid-July in the spring season and late September or mid-October in the fall season, typical of polycyclic epidemics. The Gompertz, logistic and monomolecular growth models were fitted to disease severity using linear regression and parameter estimates used to compare the effects of fungicide treatment and cucurbit host type on disease progress. The Gompertz and logistic models were more appropriate than the monomolecular model in describing temporal dynamics of cucurbit downy mildew, with the Gompertz model providing the best description for 34 of the 44 epidemics examined. Fungicide treatment and host type significantly (P < 0.0001) affected standardized area under disease progress curve (sAUDPC), final disease severity (Final DS) and weighted mean absolute rates of disease progress (ρ), with these variables, in most cases, being significantly (P < 0.05) lower in fungicide treated plots than in untreated control plots. Except in a few cases, sAUDPC, Final DS and ρ were lower in cases where chlorothalonil was alternated with fluopicolide or propamocarb than in cases where chlorothalonil alternated with cymoxanil or when chlorothalonil was applied alone. These results characterized the temporal progress of cucurbit downy mildew and provided an improved understanding of the dynamics of the disease at the field level. Parameters of disease progress obtained from this study could serve as inputs in simulation studies to assess the efficacy of fungicide alternation in managing fungicide resistance in this pathosystem.}, number={3}, journal={PLANT DISEASE}, author={Kikway, Isaack and Keinath, Anthony P. and Ojiambo, Peter S.}, year={2022}, month={Mar}, pages={1009–1019} }