Effective Management of Powdery Mildew in Cantaloupe Plants Using Nighttime Applications of UV Light.

Ultraviolet light at wavelengths from 254 to 283 nm/has been reported to effectively suppress powdery mildews in several crops, including some cucurbits. Its use to suppress powdery mildew (Podosphaera xanthii) specifically in cantaloupe has not been previously reported. We evaluated the foregoing technology in cantaloupe fields for suppression of powdery mildew and possible effects on plant growth and yield. In a controlled laboratory study, greenhouse-grown cantaloupe plants were exposed to a gradient of UV-C (254 nm) doses during darkness, and the effects upon powdery mildew development and the plant were evaluated. We also evaluated the efficacy of nighttime applications of UV-C at 100 and 200 J/m2 against powdery mildew on adaxial leaf surfaces in greenhouse, high-tunnel, and open-field plantings. UV-C at the foregoing doses reduced sporulation and germination of P. xanthii conidia without damaging plants. On cantaloupe seedlings in the greenhouse, disease severity was equivalently suppressed at all doses and frequencies of applications of the light. In high-tunnel and open-field experiments, the most effective control of powdery mildew was provided by UV-C applied at 200 J/m2 twice every week, where suppression provided by UV-C was generally equal to and sometimes better than the fungicide treatment. The foregoing UV-C dose and frequency of application also provided the highest yield under field conditions, indicating that UV-C treatment is a promising technology for commercially relevant suppression of powdery mildew on cantaloupe in a variety of growing systems.

UV-Transmitting Plastics Reduce Powdery Mildew in Strawberry Tunnel Production.

Strawberry powdery mildew, caused by Podosphaera aphanis, can be particularly destructive in glasshouse and plastic tunnel production systems, which generally are constructed of materials that block ultraviolet (UV) solar radiation (about 280 to 400 nm). We compared epidemic progress in replicated plots in open fields and under tunnels constructed of polyethylene, which blocks nearly all solar UV-B, and two formulations of ethylene tetrafluoroethylene (ETFE), one of which contained a UV blocker and another that transmitted nearly 90% of solar UV-B. Disease severity under all plastics was higher than in open-field plots, indicating a generally more favorable environment in containment structures. However, the foliar severity of powdery mildew within the tunnels was inversely related to their UV transmissibility. Among the tunnels tested, incidence of fruit infection was highest under polyethylene and lowest under UV-transmitting ETFE. These effects probably transcend crop, and the blocking of solar UV transmission by glass and certain plastics probably contributes to the widely observed favorability of greenhouse and high-tunnel growing systems for powdery mildew.

Use of Ultraviolet Light to Suppress Powdery Mildew in Strawberry Fruit Production Fields.

We designed and deployed an apparatus to apply UV light for suppression of powdery mildew in open field production of strawberry. The unit was evaluated in a commercial production field for one season, and for two additional seasons in open field research plots at the University of Florida Gulf Coast Research and Education Center. The apparatus contained two 180-cm-long hemicylindrical arrays of twenty 55-W low-pressure discharge UV-C lamps (operated at 30 W; peak wavelength = 254 nm) backed by polished aluminum reflectors covering two adjacent beds of the strawberry planting. The lamp arrays were suspended within a steel carriage that was tractor-drawn through the planting at 2.3, 4.6, and 5.6 km h-1. Nighttime applications of UV-C at doses ranging from 65 to 170 J⋅m-2 either once or twice weekly provided suppression of foliar and fruit disease that was consistently equal to or better than that provided by a commercial calendar-based fungicide spray program.

Pulsed Water Mists for Suppression of Strawberry Powdery Mildew.

Powdery mildew (Podosphaera aphanis) is a destructive and widespread disease of strawberry (Fragaria × ananassa), especially when susceptible cultivars are grown in high plastic tunnels or glasshouses. Many powdery mildews thrive in humid environments but free water films on plant surfaces can inhibit conidial germination of some species. We hypothesized that P. aphanis might be directly suppressed by rain through the action of water films and meteoric water. In repeated experiments, the hydrophobic conidia of P. aphanis collected on the surface of water droplets, resulting in their removal when the droplets rolled over the leaf surfaces and fell to the ground. Meteoric water and water films also damaged conidiophores. Brief midday water mists applied in pulses lasting 1 min each four times per day were as effective as multiple fungicide treatments in suppressing powdery mildew. Rapid drying of the pulsed mists resulted in effective suppression of powdery mildew without consequent increases of fungal pathogens that might benefit from water films. The timing and duration of water sprinkling has been refined to the point where it can provide a commercially relevant degree of suppression of powdery mildew on strawberry in a high-tunnel production system.

Effect of Water Stress on Reproduction and Colonization of Podosphaera aphanis of Strawberry.

In a number of pathosystems involving the powdery mildews (Erysiphales), plant stress is associated with decreased disease susceptibility and is detrimental to pathogen growth and reproduction. However, in strawberry, anecdotal observations associate severe powdery mildew (Podosphaera aphanis) with water stress. In a 2017 survey of 42 strawberry growers in Norway and California, 40 growers agreed with a statement that water-stressed strawberry plants were more susceptible to powdery mildew compared with nonstressed plants. In repeated in vitro and in vivo experiments, we found that water stress was consistently and significantly unfavorable to conidial germination, infection, and increases in disease severity. Deleterious effects on the pathogen were observed from both preinoculation and postinoculation water stress in the host. Soil moisture content in the range from 0 to 50% was correlated (R2 = 0.897) with germinability of conidia harvested from extant colonies that developed on plants growing at different levels of water stress. These studies confirm that P. aphanis fits the norm for biotrophic powdery mildews and hosts under stress. Mild water stress, compared with a state of optimal hydration, is likely to decrease rather than increase susceptibility of strawberry to P. aphanis. We believe it is possible that foliar symptoms of leaf curling due to diffuse and inconspicuous infection of the lower leaf surfaces by P. aphanis could easily be mistakenly attributed to water stress, which we observed as having a nearly identical leaf curling symptom in strawberry.