Effect of Cucurbit Host, Production Region, and Season on the Population Structure of Pseudoperonospora cubensis in Florida.

Pseudoperonospora cubensis, the causal agent of Cucurbit downy mildew (CDM), is one of the most important diseases affecting cucurbit production in the United States. This disease is especially damaging to Florida production areas, as the state is a top producer of many cucurbit species. In addition, winter production in central and south Florida likely serves as a likely source of P. cubensis inoculum for spring and summer cucurbit production throughout the eastern United States, where CDM is unable to overwinter in the absence of a living host. Over 2 years (2017 and 2018) and four seasons (spring 2017, spring 2018, fall 2017, and fall 2018), 274 P. cubensis isolates were collected from cucurbit hosts at production sites in south, central, and north Florida. The isolates were analyzed with 10 simple sequence repeat (SSR) markers to establish population structure and genetic diversity and further assigned to a clade based on a qPCR assay. Results of population structure and genetic diversity analyses differentiated isolates based on cucurbit host and clade (1 or 2). Of the isolates assigned to clade by qPCR, butternut squash, watermelon, and zucchini were dominated by clade 1 isolates, whereas cucumber isolates were split 34 and 59% between clades 1 and 2, respectively. Clade assignments agreed with isolate clustering observed within discriminant analysis of principal components (DAPC) based on SSR markers, although watermelon isolates formed a group distinct from the other clade 1 isolates. For seasonal collections from cucumber at each location, isolates were typically skewed to one clade or the other and varied across locations and seasons within each year of the study. This variable population structure of cucumber isolates could have consequences for regional disease management. This is the first study to characterize P. cubensis populations in Florida and evaluate the effect of cucurbit host and clade-type on isolate diversity and population structure, with implications for CDM management in Florida and other United States cucurbit production areas.

Duration of Downy Mildew Control Achieved with Fungicides on Cucumber Under Florida Field Conditions.

Cucurbit production in Florida is impacted by downy mildew on a yearly basis. Cucurbit downy mildew (CDM), caused by Pseudoperonospora cubensis, is one of the most devastating cucurbit diseases and can lead to complete yield loss. Nearly continuous production of cucurbits occurs temporally throughout Florida, which puts extensive pressure on the pathogen population to select for individuals that are resistant to fungicides in use labeled for CDM. Loss of efficacy as a result of fungicide resistance developing is becoming a major concern for Florida cucurbit growers who rely on these products to manage CDM. This study was established to evaluate the field activity of 11 utilized fungicides by determining their duration of activity when applied at various intervals for the management of CDM in cucumber under Florida field conditions. By comparing levels of percent CDM control and area under the disease progress curve values, the fungicide's duration of field activity was established. Field activities were <1 week for dimethomorph and fluopicolide; 1 week for cymoxanil; 1 to 2 weeks for chlorothalonil and mancozeb; 2 weeks for ethaboxam; 1 to 3 weeks for propamocarb, cyazofamid, and ametoctradin + dimethomorph; and 2 to 4 weeks for oxathiapiprolin and fluazinam. Knowledge of duration of field activity can potentially improve the development of CDM management programs and slow the resistance selection.

Effect of spirotetramat and fluensulfone on population densities of Mesocriconema xenoplax and Meloidogyne incognita on peach.

Management of plant-parasitic nematodes (PPNs) on peach is needed for a longer period of time than is typically afforded by pre-plant fumigant nematicides. Two post-plant nematicides, spirotetramat and fluensulfone, were evaluated for control of Meloidogyne incognita and Mesocriconema xenoplax under laboratory and greenhouse conditions. In vitro assays were conducted to test the effect of spirotetramat at 0.017 and 0.026 kg a.i./ha and fluensulfone at 3.92 kg a.i./ha on the mobility of both M. incognita and M. xenoplax in 24-well plates for 24, 48, and 72 hr, compared to a water control. Both fluensulfone and spirotetramat reduced mobility of M. xenoplax, but only fluensulfone reduced the mobility of M. incognita, compared to the untreated control. In peach greenhouse trials, both spirotetramat at 0.017 kg a.i./ha and fluensulfone at 3.92 kg a.i./ha reduced M. incognita numbers by 62 and 77% at 40 d after inoculation (DAI), respectively; neither chemical reduced populations at 70 DAI. Fluensulfone reduced M. xenoplax numbers by 84, 94, and 96% at 30, 60, and 90 DAI, respectively. No effects were observed for spirotetramat on M. xenoplax. At 40 DAI, dual applications of spirotetramat 30 d apart reduced M. incognita numbers by 58 and 54% for both 0.017 and 0.026 kg a.i./ha rates, respectively; no reductions were observed at 70 DAI. No effect was seen for a dual application of spirotetramat on M. xenoplax. These post-plant nematicides may provide additional options for management of PPNs on peach.Management of plant-parasitic nematodes (PPNs) on peach is needed for a longer period of time than is typically afforded by pre-plant fumigant nematicides. Two post-plant nematicides, spirotetramat and fluensulfone, were evaluated for control of Meloidogyne incognita and Mesocriconema xenoplax under laboratory and greenhouse conditions. In vitro assays were conducted to test the effect of spirotetramat at 0.017 and 0.026 kg a.i./ha and fluensulfone at 3.92 kg a.i./ha on the mobility of both M. incognita and M. xenoplax in 24-well plates for 24, 48, and 72 hr, compared to a water control. Both fluensulfone and spirotetramat reduced mobility of M. xenoplax, but only fluensulfone reduced the mobility of M. incognita, compared to the untreated control. In peach greenhouse trials, both spirotetramat at 0.017 kg a.i./ha and fluensulfone at 3.92 kg a.i./ha reduced M. incognita numbers by 62 and 77% at 40 d after inoculation (DAI), respectively; neither chemical reduced populations at 70 DAI. Fluensulfone reduced M. xenoplax numbers by 84, 94, and 96% at 30, 60, and 90 DAI, respectively. No effects were observed for spirotetramat on M. xenoplax. At 40 DAI, dual applications of spirotetramat 30 d apart reduced M. incognita numbers by 58 and 54% for both 0.017 and 0.026 kg a.i./ha rates, respectively; no reductions were observed at 70 DAI. No effect was seen for a dual application of spirotetramat on M. xenoplax. These post-plant nematicides may provide additional options for management of PPNs on peach.