Greenhouse Evaluation of Products That Induce Host Resistance for Control of Scab, Melanose, and Alternaria Brown Spot of Citrus.

Products that induce disease resistance in plants were evaluated on potted seedlings of rough lemon for citrus scab, caused by Elsinoe fawcettii; grapefruit for melanose, caused by Diaporthe citri; and Dancy tangerine for Alternaria brown spot caused by Alternaria alternata pv. citri. Plants were pruned to a single stem with mature leaves and treated at bud break or various times thereafter. New foliage was inoculated and subsequently evaluated for disease severity. Oxycom, Nutriphite, Messenger, Goemar H11, Serenade, ReZist, ProPhyt, Aliette, Actigard, and KeyPlex were evaluated and compared with benomyl or strobilurin fungicides as standards. Most products reduced disease severity compared with the untreated control, but were less effective than standard fungicides. The most generally effective products were ReZist and Actigard, those that contain or produce phosphorous acid (Aliette and Nutriphite), and a bacterial preparation (Serenade). Oxycom and Messenger controlled scab well in some tests. Products that induce host resistance may be useful for disease control in citrus in an integrated program with standard fungicides.

Influence of Environmental Factors on Severity of Citrus Scab and Melanose.

Citrus scab, caused by Elsinoe fawcettii, and melanose, caused by Diaporthe citri, produce external blemishes on citrus fruit, reducing acceptability of the fruit for the fresh market. In laboratory studies, rough lemon seedlings and grapefruit seedlings were inoculated with conidia of E. fawcettii and D. citri, respectively, and exposed to a range of temperatures and durations of leaf wetness. Scab was most severe at temperatures from 23.5 to 27°C and much less severe at 17, 20, 30, or 32°C. A leaf wetness duration of 4 h was sufficient for some infection, but 12 h of leaf wetness were needed for maximum infection with scab. Melanose was equally severe at 24 and 28°C, moderate at 20°C, and low at 32°C. Melanose infection was minimal with 4 h of leaf wetness, moderate with 8 to 16 h, and reached maximum levels at 24 h or more of leaf wetness. In field studies, grapefruit seedlings with new shoots were placed beneath trees weekly, and disease severity was evaluated in relation to environmental factors. Melanose severity increased sharply with an increase in total weekly rainfall, leaf wetness durations of greater than 80 h per week, and average temperatures above 22°C.

Epidemiology and Control of Citrus Greasy Spot in Different Citrus-Growing Areas in Florida.

Greasy spot, caused by Mycosphaerella citri, causes defoliation and yield losses on grapefruit in all areas of Florida, but is more severe in southwest Florida and the east coast than in central Florida. The amount of leaf litter, numbers of ascospores produced, and severity of greasy spot on trap plants were monitored throughout 1997 and 1998 in Immokalee (southwest Florida) and Lake Alfred (central Florida). Leaf litter and ascospore production were greatest from March to July in both locations, with little litter and few ascospores thereafter. Ascospore production occurred earlier in Immokalee than in Lake Alfred in both years. Disease on trap plants was moderate to severe throughout the year except from November to February. Large numbers of ascospores produced early in the year when conditions were less favorable resulted in the same disease severity levels as low numbers of ascospores produced later in the year when environmental conditions were favorable. Greater greasy spot severity in southwest Florida, compared with central Florida, is more likely due to higher rainfall and warmer winter temperatures than to differences in time of infection. Single annual copper fungicide applications were made each month from April to August in 1998 and 1999 in LaBelle (southwest Florida), Ft. Pierce (east coast), and Lake Alfred to determine the most effective time of application. Two two-spray treatments, May + July and June + August, were also evaluated in 1999. A single copper fungicide application in June provided the most consistently effective control across all locations. The June + August two-spray treatment was very effective in disease control, but usually no better than a well-timed single application.

Evaluation of Postinfection Control of Citrus Scab and Melanose with Benomyl, Fenbuconazole, and Azoxystrobin.

Citrus scab, caused by Elsinoe fawcettii, and melanose, caused by Diaporthe citri, are serious diseases of citrus in Florida that reduce the exterior quality of fruits produced for the fresh market. The availability of fungicides with postinfection activity could increase the flexibility of growers in managing these diseases. The effectiveness of benomyl at 0.96 g a.i./liter, fen-buconazole at 120 mg a.i./liter, and azoxystrobin at 235 mg a.i./liter was evaluated on rough lemon seedlings for scab and on grapefruit for melanose. Potted seedlings in the greenhouse were pruned to stimulate formation of uniform susceptible shoots, which were inoculated and kept moist for 16 h. Fungicides were applied prior to inoculation and at various intervals from 16- to 96-h postinoculation and subsequently evaluated for disease intensity. All three fungicides were effective as preinoculation sprays for scab control. Benomyl was effective if applied up to 72 h after inoculation, and fenbuconazole and azoxystrobin were effective if applied within 16 to 48 h after inoculation. For melanose, azoxystrobin was highly effective as a preventive spray but benomyl and fenbuconazole were not. None of the fungicides was very effective for melanose control as postinfection treatments. Use of postinfection sprays under field conditions appears to be promising for scab control but not for melanose control.

Environmental Factors Affecting the Severity of Alternaria Brown Spot of Citrus and Their Potential Use in Timing Fungicide Applications.

Alternaria brown spot affects many tangerines and their hybrids, causing lesions on leaves, twigs, and fruit resulting in reduced yield and fruit quality. Field studies were conducted in a severely affected Minneola tangelo grove in central Florida from 1996 to 1998 to determine the environmental factors associated with infection of field trees and potted trap plants. Conidial production peaked following large flushes of new leaves, which were heavily infected. Most infections occurred during the summer rainy season, but trap plants became infected nearly every week of the year. When plants were exposed for 1-week periods, linear regression analysis indicated that disease severity on trap plants was positively related to the amount of rainfall, duration of leaf wetness, and average daily temperatures, and negatively related to the number of conidia trapped. Similar relationships occurred with trap plants exposed for 24-h periods on 141 different dates, except that temperature was not a significant factor. Nevertheless, these factors individually or combined in stepwise multiple regressions explained only a low percentage of the variability in disease severity with both weekly and daily trap plant sampling. When daily environmental data were categorized as: (i) rain versus no rain, (ii) <10 h or >10 h leaf wetness duration, and (iii) average temperature <20°C, 20 to 28°C, and >28°C, relationships to disease severity on trap plants were clearer. Disease severity on days with rain was nearly double that of days without rain, but considerable infection occurred on days with >10 h leaf wetness duration and no rain. Infection was greatest on days with temperatures of 20 to 28°C and slightly less at lower or higher temperatures. A point system, called the ALTER-RATER, was designed whereby each day would be assigned a severity value according to the prevailing environmental conditions. A fungicide application would be made after a predetermined number of points had accumulated. Simulated spray programs based on accumulation of 50, 75, 100, and 150 points from historical weather data at several locations indicated that from 8 to 15, 6 to 8, 5 to 6, or 3 to 4 sprays, respectively, would be needed depending on year and location in Florida. Such a weather-based control system could reduce the number of fungicide applications and improve control of Alternaria brown spot of tangerine.