L. L. Wilson

Evaluation of a disease warning system for downy mildew of grapes.

An electronic warning system for grape downy mildew- based on models for the infection of leaves of Vitis lambrusca, production of sporangia by Plasmopara viticola in lesions, and sporangial survival-was tested over 7 years in Ohio. Grapevines were sprayed with metalaxyl plus mancozeb (Ridomil MZ58) when the warning system indicated that environmental conditions were favorable for sporulation and subsequent infection. Over the 7 years, plots were sprayed from one to four times according to the warning system, and from four to 10 times according to the standard calendar-based schedule (depending on the date of the initiation of the experiment). The warning system resulted in yearly reductions of one to six sprays (with median of three sprays). Disease incidence (i.e., proportion of leaves with symptoms) in unsprayed plots at the end of the season ranged from 0 to 86%, with a median of 68%. Incidence generally was very similar for the warning-system and standard-schedule treatments (median of 7% of the leaves with symptoms), and both of these incidence values were significantly lower (P < 0.05) than that found for the unsprayed control, based on a generalized-linear-model analysis. Simplifications of the disease warning system, where sprays were applied based only on the infection or sporulation components of the system, were also effective in controlling the disease, although more fungicide applications sometimes were applied. Effective control of downy mildew, therefore, can be achieved with the use of the warning system with fewer sprays than a with a standard schedule.

Effects of temperature on sporulation and latent period of Colletotrichum spp. infecting strawberry fruit.

Effects of temperature on sporulation of Colletotrichum acutatum, C. gloeosporioides, and C. fragariae, causes of anthracnose of strawberry, were determined in controlled-environment studies. Detached immature fruit were inoculated with a conidial suspension and incubated up to 36 days at constant temperatures of 5, 10, 15, 20, 25, 30, and 35°C. Latent period (time to first sporulation) depended on temperature and ranged from 2 to 3 days at 25°C to 6 to 17 days at 5°C. C. acutatum had a shorter latent period than the other species at 5 and 10°C; at higher temperatures, latent periods of the species were very similar. During the first 4 days of sporulation, there was an optimum-type relationship between the logarithm of conidia per fruit [log(Y)] and temperature, with maximum observed sporulation (generally 106 to 107 conidia per fruit) from 15 to 30°C. Sporulation increased over time at temperatures of 15°C and above. The greatest difference among the species was at 5 and 10°C, where tested C. acutatum isolates produced from 10 to 100 more conidia per fruit than the other species. Polynomial regression equations were used successfully to represent log(Y) as a function of temperature and incubation time. The rate of increase in sporulation over time was a function of temperature, with a predicted optimum of 22 to 26°C. Equations were validated by predicting sporulation of the three species infecting fruit attached to plants growing in controlled-environment chambers. Although the predictions tended to be slightly larger than observed, mean error [100(observed - predicted)/ observed] was only -0.7% (95% confidence interval: -2.4 to 1.0%).

Overwinter survival of Colletotrichum acutatum in infected strawberry fruit in Ohio

Colletotrichum acutatum, a causal organism for anthracnose fruit rot of strawberry, was recovered from 100% of all infected strawberry fruit exposed for 18 wk in the laboratory to constant temperatures of -12 and -30 C or six cycles of fluctuating temperatures of -12 or -30 C for 2 wk followed by 5 C for 1 wk. In a 2-yr field study (1988-89, 1989-90), C. acutatum was recovered from nearly 100% of fruit located on the soil surface or 5-8 cm below the soil surface after exposure to winter conditions for 3 mo (November through January), but recovery of C. acutatum decreased with time after 3 mo (.)

Effects of application of fungicide during the dormant period on phomopsis cane and leaf spot of grape disease intensity and inoculum production

Efficacy of application of the fungicides calcium polysulfide or fixed copper during the dormant period on control of Phomopsis cane and leaf spot of grape (Vitis spp.), caused by Phomopsis viticola, was examined under field conditions during the 2003 and 2004 growing seasons in Ohio. Dormant-period fungicide applications were made either in the fall (after leaf drop and periderm tissue formation on the first-year canes, mid-November), or spring (at bud-swell, mid-April), or both. Disease incidence and severity on leaves and internodes were examined. In addition, effects of dormant-period application on sporulation of P. viticola were determined by examining the number of conidia in rain-splashed water in the spring and formation of mature pycnidia on cane sections in the winter. Fall-and-spring and spring applications of calcium poly-sulfide provided 12 to 88% reduction in disease intensity (incidence or severity), whereas calendar-based protectant mancozeb applications reduced overall disease intensity by 47 to 100%. Fixed-copper applications did not provide a consistent reduction of the disease. Fall applications of dormant-period fungicide provided little or no effect by itself. There was a significantly lower number of conidia observed in collected splashed rain water from vines treated with fall-and-spring applications of calcium polysulfide than in rain water from nonsprayed vines. Fall-and-spring and spring applications of calcium polysulfide provided a significant reduction in the number of mature pycnidia formed on incubated cane sections compared with the nonsprayed control (5 versus 10 pycnidia/cm2), whereas fixed copper did not provide a significant reduction.

Evaluation of a Disease Warning System for Phomopsis Cane and Leaf Spot of Grape: A Field Study

A field evaluation of a warning system for Phomopsis cane and leaf spot of grape (Vitis spp.), caused by Phomopsis viticola, was conducted in Ohio over 3 years (2002 to 2004) by applying fungicides and fungicide-adjuvant combinations based on predicted infection events. Three different criteria for risk-light, moderate, and high-were evaluated with the warning system. The warning system is based on measured weather conditions (temperature and wetness duration following rain) and a model for risk of leaf and internode infection. Vines were sprayed with fungicides based on either the warning system or a calendar-based 7-day protectant program, from 2.5-cm shoot growth (Eichhorn-Lorenz [E-L] stage 7) to the end of the broom (E-L stage 27). Fungicides were tested with or without an adjuvant (JMS Stylet-Oil or Regulaid). In the controls, the mean percentage of leaves and internodes with infections ranged from 36 to 100%, the number of lesions per leaf ranged from 1 to 28, and percentage of internodes covered by lesions ranged from 1 to 12%. Both the calendar-based protectant treatment (based on use of mancozeb) and the warning system treatment based on spraying in response to light or moderate predicted infection events (especially with mancozeb + Regulaid) resulted in significantly less disease incidence and severity compared with the controls. The mean percent control (relative difference in disease between a treatment and the control) was higher for the protectant schedule (˜55% and ˜80% for incidence and severity, respectively, based on application of mancozeb) than for the warning system (˜36% and ˜60% for incidence and severity, respectively, based on application of mancozeb + Regulaid), but there were two to three times more fungicide applications with the protectant schedule than with the warning system.