Baseline sensitivities of Monilinia oxycocci to propiconazole and fenbuconazole

Control of cranberry cottonball disease has relied on the sterol demethylation inhibitor (DMI) fungicides Funginex® (triforine) and Orbit® (propiconazole) since 1982. In other crop systems, some pathogens, including species related to Monilinia oxycocci (the cottonball fungus), have become resistant to DMI fungicides after several years of use. In this study, cottonball management strategies were developed to reduce the risk of fungicide-resistant M. oxycocci evolving in Wisconsin. Field studies in 1996-1997 showed that sprays made during shoot elongation had little or no impact on the percent cottonball infected fruit at harvest. Skipping shoot elongation sprays, and just spraying during bloom, was as effective in reducing the percentage of cottonball infected fruit as spraying during both shoot elongation and bloom. A DMI fungicide (fenbuconazole) and the non-DMI fungicides (azoxystrobin and cyprodinil) were effective in controlling cottonball when used alone or mixed with Orbit®, but an inducer of systemic acquired resistance in plants (benzothiadiazole) had no effect. In 1998, cottonball disease pressure was too low at our field site to gather meaningful efficacy data. None of the treatments affected yield, fruit retention, or berry weight compared to the controls. Sensitivity of M. oxycocci, the cottonball pathogen, to fenbuconazole and propiconazole was tested in vitro by comparing the distributions of ED₅₀ values of populations collected from three sites that differed in previous exposure to fungicides. Median ED₅₀ values for fenbuconazole were significantly greater at sites where sterol demethylation inhibitor fungicides had been used compared to a site where fungicides had never been used, but median ED₅₀ values for propiconazole did not differ among sites. There was no correlation between the sensitivities to fenbuconazole and propiconazole. The data will form the basis of recommendations aimed at delaying the onset of fungicide resistance, and provide a baseline for monitoring resistance to fenbuconazole and propiconazole in populations of M. oxycocci in the future.

Cottonball, caused by the fungus Monilinia oxycocci, is the most important disease of cranberry in Wisconsin during the growing season. Control of cottonball has relied on the sterol demethylation inhibitor (DMI) fungicides triforine (Funginex®) and propiconazole (Orbit®) since 1982. The sensitivity to DMIs of some fungal pathogens, including species related to M. oxycocci, has decreased after continuous and exclusive use over periods of about 10 years. Fungi resistant to one DMI (e.g., triforine) are almost always resistant to other DMIs (e.g., propiconazole), a phenomenon known as cross resistance. Thus, the cranberry industry in Wisconsin is faced with the risk of M. oxycocci becoming resistant to the only fungicide available for controlling cottonball.

In 1997, up to four applications of propiconazole were applied to 3,100 acres, or 22% of the total cranberry acreage in Wisconsin at a cost of $186,000 to the industry, excluding labor and fuel. If M. oxycocci became resistant to propiconazole, direct losses would be suffered by growers through reduced yields and by handlers through increased sorting costs. By determining which sprays (shoot elongation [budbreak] or bloom) are more critical in reducing cottonball incidence at harvest, we may be able to reduce the total number of applications. This strategy has been effective in delaying the onset of resistance to pesticides in other systems. Also important in delaying the onset of resistance is broadening the spectrum of fungicide modes of action by introducing chemistries unrelated to the DMIs. Finally, having baseline sensitivity data will provide a standard to which fungal sensitivity can be compared in the future, should there be evidence of fungicide resistance in the field. Managing a fungicide resistance crisis would almost certainly be more costly than averting the crisis. Averting such a crisis is the primary aim of the proposed research.

Objectives 1 and 2. Preliminary work by our group and published results of Steve Jeffers suggested that applying fungicides during both shoot elongation and bloom does not decrease cottonball incidence at harvest compared to making applications only during bloom. Field experiments were aimed at identifying applications that could be eliminated while achieving acceptable control and comparing the efficacy of DMIs (propiconazole and fenbuconazole), and unrelated compounds (azoxystrobin, cyprodinil, and benzothiadiazole). The following 10 treatments were applied to 2.0 m² plots (8 repetitions; randomized block design) in a bed of the cultivar Searles in northern Wisconsin.

a1^st application when new shoot growth is 1-3 cm long; 2^nd application 7-10 days later.

b1^st application at 10-20% bloom; 2^nd application 7-10 days later.

 

Primary infection was almost absent in the plots and was not rated. Secondary infection was rated by determining the percent infected berries per total berries in four 270 cm² randomly selected areas in each plot. Yield and yield components, including mass of marketable fruit, individual berry mass, number of flowers per flowering upright, and fruit retention, were compared among treatments. Data were analyzed by standard analysis of variance.

Objective 3. Approximately 50 isolates of M. oxycocci were collected from each of three different sites: a site where neither DMIs nor other fungicides had ever been used (site 1); a site where DMIs had been used extensively since 1989 (site 2), and a site where DMIs had been the primary but not sole fungicide applied (site 3). Fungal colonies were grown on potato dextrose agar (PDA). After 10-14 days fungal hyphae and spores were suspended in sterile, distilled water and 20 m l aliquots of the suspension transferred to PDA amended with propiconazole or fenbuconazole at various concentrations. In vitro testing of azoxystrobin and cyprodinil is prone to error, so we did not test these fungicides. Fungal colony diameters on the fungicide plates were measured after 7-10 days. ED₅₀ values were calculated by regressing relative growth (colony diameter on amended medium divided by the diameter on unamended medium multiplied by 100) against the log₁₀ of the fungicide concentration. Frequency distributions for ED₅₀ values for each of the three populations were constructed to determine i) the baseline sensitivities to each of the fungicides for populations that have never been exposed to fungicides; ii) whether the populations exposed to DMIs have shifted toward DMI resistance; and iii) whether cross resistance between the two DMIs exists.

Results / Discussion:

Objectives 1 and 2, fungicide efficacy and timing. In 1998, cottonball disease pressure was too low at our field site to gather primary infection data. Secondary infection was greater in the controls and benzothiadiazole treated plots, but disease incidence was so low that these differences are not biologically meaningful. None of the treatments affected yield, fruit retention, or berry weight compared to the controls. These data, together with data from previous experiments will be used to support registration of new fungicides for use against cottonball.

Table 1. Effects of fungicides and spray schedule on infection of cranberry by Monilinia oxycocci and on yield and selected yield components in northwestern Wisconsin, 1998.