Brown rot - Practical Fungicide Resistance Issues

Brown rot of peach

An integrated approach is currently being used to control brown rot, including sanitation practices, cultural methods, and chemical control measures. Nonetheless, management still depends heavily on the application of two or three fungicide applications in the final 2 to 3 weeks before harvest. Since the early 1980s these applications have relied primarily on DMI fungicides but resistance development in the pathogen is forcing growers to alternate between chemical classes and/or use fungicide mixtures. However, our research indicates that alternating DMIs at the current label rate and QoIs may not be sustainable. Growers are encouraged to use the resistance monitoring program 'Profile' to determine if BZIs can be used at their specific location and if the DMI rate needs to be increased.

Demethylation inhibitor (DMI) fungicides are very effective against brown rot and -as mentioned above- have been used for a long time in the Southeast. Examples are propiconazole, fenbuconazole, tebuconazole, and metconazole. Resistance was first documented in Georgia in 2004 and has since been reported in South Carolina and other states. The good news is that increasing the rate of DMI fungicides (which is allowed for some but not all DMIs) still control resistant populations

Benzimidazole (BZI) fungicides are effective against brown rot and were in fact utilized extensively when they were first introduced in the 1970s. However, resistance developed within a few years of their introduction to the market and caused a major brown rot outbreak in 1984, where Benlate and Topsin M were extensively applied, often without a mixing partner. It was a very wet year following a major freeze in 1983 with little to no brown rot control because not enough fruit was on the trees (just enough for brown rot to develop). The BZIs provide a classic example of single-step or qualitative resistance, resulting in essentially no control of M. fructicola within a short time period. Our research shows that despite infrequent use of BZIs in the Southeast, resistant populations have persisted in South Carolina and Georgia.

Since the 1984 outbreak, BZI's have been used sparingly (once a year the most) and ONLY in combination with a compound having a different mode of action such as captan. Unfortunately, captan is associated with inking of the peach skin and therefore avoided for late season applications. Another uncertain factor are current formulations of benzimidazoles. Some specialists believe that Benlate (which is not available any longer) was superior in performance compared to current formulations other insist that Topsin M for example at the high rate will do just fine. Research is needed to document what rate of current formulations will do the job (most tests in the past were done with Benlate) and what -if any- mixing partner can be used safely. One thing is for sure, we will have to find a way to incorportate the BZIs in current southeastern spray programs.

Chlorothalonil and dicarboxamides are not registered after bloom

Anilinopyrimidines were not sufficiently effective in experimental preharvest brown rot sprays conducted by Eldon Zehr in the 1990s. That was the main reason why Vangard was not registered here in the Southeast. They seem to work just fine in California (where Vangard is registered) but reports of resistance in M. fructicola to anilinopyrimidines may limit their usefulness in the future. The poor performance of anilinopyrimidines against brown rot was confirmed in 2008 spray trials (pdf) conducted on two cultivars in South Carolina.

Quinone outside inhibitors (QoIs) include azoxystrobin and pyraclostrobin (which is available only in combination with boscalid). They have been available since about 2000 (azoxystrobin) for brown rot control. They are single-site inhibitors as well and thus prone to resistance development. In a recent study, we showed that DMI-resistant M. fructicola isolates can be predisposed to developing QoI resistance and in fact, we have already found populations with isolates exhibiting reduced sensitivity to azoxystrobin (paper should come out in 2009). We predict that in the advent of DMI resistance in M. fructicola and the absence of viable alternatives growers will increase use of QoIs, with associated increase in selection pressure for resistance. Resistance to QoIs is likely to occur rapidly as a single-step resistance, similar to that observed with the BZIs

Fungicides having multiple sites of action, which are not prone to development of insensitivity, are not available. While sulfur and captan are labeled for use against pre-harvest brown rot, these products are not effective enough when utilized for pre-harvest brown rot management. Captan may also cause inking, if applied in concentrated sprays.