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  • Author or Editor: Christian A. Wyenandt x
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The effects of two pumpkin cultivars and five fungicide programs on cucurbit powdery mildew development and yield were evaluated in southern New Jersey from 2005 to 2007. Each year, five separate fungicide programs were applied to powdery mildew-tolerant cv. Magic Lantern or powdery mildew-susceptible cv. Howden pumpkin. The five fungicide programs applied season-long (10 applications per program) included: 1) protectant fungicides only: manzate + sulfur [Fungicide Resistance Action Committee (FRAC) codes M3 + M2] alternated weekly with maneb + copper hydroxide (FRAC codes M3 + M1); 2) standard program: chlorothalonil + myclobutanil (FRAC codes M5 + 3) alternated with azoxystrobin (FRAC code 11); 3) intensive program: maneb + myclobutanil (FRAC codes M3 + 3) alternated with [famoxadone + cymoxanil] (FRAC codes 11 + 27); 4) FRAC code 3 weekly: chlorothalonil + myclobutanil (FRAC codes M5 + 3) alternated with myclobutanil (FRAC code 3); and 5) FRAC code 11 weekly: chlorothalonil + azoxystrobin (FRAC codes M5 + 11) alternated with azoxystrobin (FRAC code 11). In each year, there were no significant interactions between the fungicide program and cultivar. In each year, area under disease progress curve values were highest when a FRAC code 11 fungicide was applied weekly compared with a FRAC code 11 fungicide applied in a weekly rotation with a FRAC code 3 fungicide or a FRAC code 3 fungicide applied weekly.

Visual examination of leaves at the end of each production season revealed there were no significant differences in powdery mildew development on the top (adaxial) or bottom (abaxial) sides of leaves in untreated subplots. Powdery mildew development was lower on the bottom sides of leaves when a Fungicide Resistance Action Committee (FRAC) code 3 fungicide was applied weekly compared with a FRAC code 11 fungicide applied weekly or when a FRAC code 3 fungicide was rotated weekly with a FRAC code 11 fungicide in each year of the study. There were no significant differences in total number of harvested fruit, number of harvested orange fruit, average weight of orange fruit, or percentage of harvested orange fruit between fungicide programs in each year of the study. Results of this study, based on arcsine-transformed area under disease progress curve (AUDPC) values and top and bottom leaf surface ratings, suggest that the weekly use of the FRAC code 11 fungicide lead to the development of practical resistance in the field population of cucurbit powdery mildew. Rotating a FRAC code 11 and FRAC code 3 fungicide weekly resulted in lower AUDPC values and powdery mildew development on the bottom side of leaves in 2 of 3 years of this study. However, based on AUPDC values and leaf rating values, the level of control obtained with the high-risk FRAC code 3 fungicide was less during each consecutive year. The immediate erosion of control (i.e., qualitative resistance) as observed with the FRAC code 11 fungicide or the gradual decline of control (quantitative resistance) as observed with the FRAC code 3 fungicide over three growing seasons shows the importance of being able to detect and understand the mechanisms of practical resistance development. This understanding will allow appropriate fungicide control recommendations to be made in a timely (i.e., real-time) manner. Importantly, fungicide resistance is most likely to develop on the bottom side (abaxial) of pumpkin leaves when effective, low-risk (nonmobile) fungicides (FRAC code M numbers) are tank-mixed with high-risk fungicides in cucurbit powdery mildew control programs. Tank-mixing fungicides that have a high risk for resistance development with protectant fungicides that have a low risk for resistance development remains critically important when controlling cucurbit powdery mildew and reducing the potential for fungicide resistance development. This is the first report of cucurbit powdery mildew developing practical resistance to a FRAC code 11 and FRAC code 3 fungicide in New Jersey.

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The development of septoria leaf spot in processing tomatoes grown on conventional (bare soil) beds or beds with chemically or mechanically killed winter rye (Secale cereale L.) and hairy vetch (Vicia villosa Roth) cover crop mulch with or without fungicide was examined. The two fungicide treatment programs included fungicide applied weekly (7 d) and a no fungicide control. In mulch bed systems without fungicide, septoria leaf spot caused ≈50% defoliation 10 and 28 d later in 1997 and 1998 than in the conventional system, respectively. In both years, area under disease progress curve (AUDPC) values for septoria leaf spot development were lower with the presence of a chemically or mechanically killed mulch compared with the conventional bed system when no fungicide was applied. In 1997, there were no significant differences in AUDPC values for septoria leaf spot development when fungicide was applied weekly. In 1998, AUDPC values were lower in both mulch systems compared with the conventional bed system when fungicide was applied weekly. At harvest in both years, defoliation was highest in the no fungicide control treatment. In 1997, marketable yield was significantly higher in both mulch systems compared with the conventional bed system. Conversely, in 1998, marketable yield was significantly higher in the conventional bed system than in either mulch bed system.

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Sweet basil (Ocimum basilicum L.) is among the most widely popular and economically important culinary herbs. Worldwide production of sweet basil has been threatened by a newly emerging disease, downy mildew (Peronospora belbahrii). Although tolerance and resistance have been identified in other Ocimum species, the traditional sweet basils all have been reported to be highly susceptible. There is an urgent need for evaluation of basil germplasm to identify sources of host resistance to P. belbahrii within Ocimum spp. and especially among O. basilicum species. In searching for genetic resistance, we developed a rapid approach to screen and evaluate downy mildew response at the cotyledon and true leaf growth stages under controlled environmental conditions. To confirm the reliability and reproducibility of this screening method, an experiment was conducted in which three basil species (Ocimum basilicum, sensitive; O. xcitriodorum, tolerant; and O. americanum, resistant to basil downy mildew) were evaluated for response to downy mildew inoculations at three growth stages. Disease incidence (DI) at the cotyledon growth stage was equal to or greater than true leaf growth stages for all species indicating that cotyledon response to downy mildew inoculations is a viable marker for predicting true leaf stage resistance. This approach was then used to screen 36 USDA-NPGS O. basilicum accessions at cotyledon and first true leaf growth stages to identify promising downy mildew-resistant breeding lines. Thirty accessions were susceptible at both growth stages (DI = 1.0). Four accessions exhibited little or no sporulation at either growth stage (DI less than 0.06), three of which showed other symptoms including chlorosis and necrosis. One accession, PI 652053, demonstrated no signs or symptoms but differed greatly from other accessions in regard to leaf morphology and habit. Results show that a resistant, mature plant can be identified at the cotyledon growth stage, providing a robust, low-input approach to identify promising downy mildew-resistant breeding material. Field evaluations of basils under high downy mildew pressure confirmed the applicability of this new screening approach to identify resistance to basil downy mildew.

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Downy mildew, caused by Peronospora belbahrii, is a new disease of basil (Ocimum spp.) in the United States. In 2009, different basil species, cultivars, and advanced breeding lines of sweet basil (30 in total) were evaluated for susceptibility to basil downy mildew in field trials in southern and northern New Jersey. Popular commercial sweet basil cultivars such as Martina, Nufar, and Poppy Joe were among the most susceptible to downy mildew. Symptoms and sporulation of P. belhahrii on Ocimum ×citriodorum and O. americanum cultivars were present but far less than on most O. basilicum cultivars evaluated. The cultivars Spice, Blue Spice, and Blue Spice Fil were the least susceptible to basil downy mildew with no visible symptoms. Similar results were observed in both field trials. This is the first report of potential resistance in Ocimum spp. to basil downy mildew. Observations from this study show that the development of resistant cultivars may be possible. Selection criteria such as foliar morphology, plant architecture as well as the presence of secondary metabolites are being examined as potential traits for developing downy mildew resistant basil cultivars.

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Sweet basil (Ocimum basilicum) is the most economically important culinary herb in the United States. In 2007, a new disease, basil downy mildew (BDM), caused by the oomycete pathogen Peronospora belbahrii, was introduced into the United States and has since caused significant losses in commercial basil production. Although no commercial sweet basils available are resistant to P. belbahrii, other species of Ocimum have exhibited potential tolerance, resistance, or both. The objectives of this work were to determine if leaf morphological characteristics including stomata density and leaf curvature correlated with infection of plants by P. belbahrii, and thus could be used as selected characters in plant breeding. In 2011, 20 Ocimum cultivars including sweet (O. basilicum), cinnamon (O. basilicum), clove (O. basilicum), citrus (Ocimum ×africanum syn. Ocimum citriodorum), spice (Ocimum americanum syn. Ocimum canum), and holy basils (Ocimum tenuiflorum syn. Ocimum sanctum) were evaluated for susceptibility to downy mildew. Sweet basils were determined to be the most susceptible; cinnamon, clove, and Thai types were moderately susceptible; and citrus, spice, and holy types were least susceptible to downy mildew. Using those same 20 Ocimum species and cultivars, stomata length and density and leaf curvature were measured and correlated with downy mildew incidence and severity. In general, basil species with higher stomatal densities had higher downy mildew incidence and severity. High stomatal densities were mainly found in the sweet, cinnamon, and clove basils. Citrus and spice species with longer stomatal lengths generally exhibited lower downy mildew incidence. Holy basil, the least susceptible of all Ocimum sp. to P. belbahrii evaluated in this study, had the greatest stomatal density and shortest stomatal length. Some sweet basil cultivars with the highest downy mildew incidence also had the greatest downward leaf curvature, whereas other sweet basil cultivars with moderate downy mildew incidence had leaves that were nearly flat or curved upward. Holy, citrus, and spice basils with low downy mildew incidence had leaves that were nearly flat or curved upward. This study suggests that leaf curvature and stomatal density and length affect downy mildew development and sporulation. Considerations of these leaf morphological characteristics may be useful phenotypic traits in breeding for downy mildew resistance in Ocimum.

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Fusarium wilt of basil (FOB), caused by Fusarium oxysporum f. sp. basilici, is an economically damaging disease of field- and greenhouse-grown sweet basil. Growers have observed a resurgence of FOB and susceptibility in FOB-resistant cultivars. Because currently available chemical, biological, and cultural control methods are costly, unsustainable, ineffective, or challenging to implement, new strategies of FOB control are needed. Cold plasma is becoming an increasingly important experimental technology in the food and agricultural industry for pathogen decontamination. To understand the effect of cold plasma treatment on FOB incidence and severity, experiments were conducted by treating FOB mycelium, inoculated sweet basil seedlings, and seeds with various experimental cold plasma treatment devices, all using helium as a feed gas. Initial results indicated that while the cold plasma jet treatment did not result in a significant reduction in mean mycelial growth rate or virulence of the pathogen, direct cold plasma jet treatments on seedlings, as well as a cold plasma dielectric barrier discharge treatment on seeds, did exhibit varying efficacies against FOB. Control of FOB appeared to be strongly dependent on the exposure time to cold plasma. These findings can aid in the standardization of a cold plasma treatment for the commercial basil seed and transplant industry.

Open Access