Fungicide Resistance in Botrytis: An Update from PA and MD Strawberry Fields

Fungicides remain a major tool for disease control. Still, the need for frequent sprays coupled with a high risk of resistance development in Botrytis sets the stage perfectly for the pathogen to prevail over many existing fungicides.  

To better understand current resistance issues in Botrytis from fields with diverse management practices and climatic conditions, samples were collected from seven Pennsylvania farms and five Maryland farms in the Spring of 2023. Strawberry flowers are considered the gateway for Botrytis infections, and the pathogen typically remains latent within developing fruit tissue until the fruit ripens.

Figure 1. A) Dead strawberry flowers; B) Botrytis from the flowers after incubation; C) A set of 24 wells for fungicide resistance detection. Photos: Dr. Mengjun Hu, University of Maryland

Our samples were largely comprised of dead flowers damaged by spring frosts (Figure 1A). Collecting samples of this type allows us to get ‘fresh’ Botrytis strains at the beginning of each new season in growers’ fields. Those strains could be a result of latent infections in transplants (plasticulture), carry-over from previous crops (matted-row), or new infections from the environment because of this disease’s airborne nature (Figure 1B). Strains obtained were then subject to resistance tests by transferring them to 24-well plates containing artificial growth media amended with fungicides commonly used in strawberry fields. The resistance profiles of each Botrytis strain were then determined based on the presence or absence of their growth on plates (Figure 1C). Botrytis evolves quickly due to its short life cycle and massive spore production. Thus, the strains, even from the same fields, can have diverse genetics and growth characteristics.      

To what extent were the strains resistant to different fungicides? A total of 231 Botrytis strains were obtained from both plasticulture and matted-row production systems. Fungicides tested including cyprodinil (one of the two active ingredients in Switch), pyraclostrobin (Cabrio), fenhexamid (Elevate), thiophanate-methyl (Topsin M), iprodione (Rovral), fludioxonil (the other active ingredient in Switch), boscalid (one of two active ingredients in Pristine), isofetamid (Kenja), penthiopyrad (Fontelis), and pydiflumetofen (one of the two active ingredients in Miravis Prime). Those fungicides represent 7 different chemical groups commonly involved in our spray programs. Resistance was detected to all of the fungicides tested with varying frequencies (Figure 2). Overall, high frequencies of resistance were detected to cyprodinil (which is in chemical class 9), pyraclostrobin (in chemical class 11), and fenhexamid (in chemical class 17) whereas low frequencies of resistance were detected to all group 7 fungicides such as Kenja and Fontelis containing isofetamid and penthiopyrad, respectively. Moreover, the majority of the isolates were resistant to more than one fungicide group (or chemical class) simultaneously, with resistance to 2, 3 ,4, 5, or 6 fungicide groups occurring simultaneously in some isolates.  In figure 3 below, CCR stands for chemical class resistance; a 2CCR isolate means the isolate is resistant to two different chemical groups, and so on.

Figure 2. Frequency of Botrytis isolates with resistance to different fungicides

Figure 3. Frequency of Botrytis isolates with multi-fungicide resistant phenotypes

Were there differences between matted-row and plasticulture fields? Based on our current data, it seems that matted-row fields had less resistance. As shown in Figures 2 and 3, both the frequency of resistance and extent of chemical class resistance among fungicide groups were detected less in isolates from matted row fields. Plasticulture isolates had more resistance to Topsin M, Rovral, and Switch compared to matted-row isolates. Additionally, more plasticulture isolates were found to be resistant to 5 or 6 different chemical groups, and a few of them were even resistant to all seven groups tested. These differences may be attributed to our assumption that fungicide sprays are applied fewer times in matted-row strawberries. For the plasticulture system, transplants typically have been treated with Botrytiscides multiple times at nurseries before they were planted in the growers’ fields. As mentioned, Botrytis strains we obtained from flowers could also come from those transplants. However, as noted, we have only one year’s data, which may not completely represent the overall resistance profile.   

How much resistance will significantly impact control efficacy? Some studies have shown that fungicide efficacy will drop by as much as 30% if only 10% of isolates are resistant in the population. Once the resistant population reaches 50% or more, efficacy could be decreased by more than 60%.

How do we slow down resistance development to avoid running out of fungicide classes? Maintaining a minimum of 7 days between fungicide applications would be a good start. If no rain is forecasted, the interval between sprays can be extended to two weeks or longer. Some disease forecast model programs, such as Cornell NEWA and the Strawberry Advisory System, are also useful for reducing chemical inputs, especially during dry seasons.

When referring to fungicides as “multi-site”, “site-specific”, or “single site”, the word “site” refers to sites of activity that the fungicide affects within the disease fungi, making it less likely that the fungi can continue to live and/or multiply.  Multi-site fungicides Captan or Thiram, which are not prone to resistance development, should be used as the backbone of spray programs. All of the other fungicides mentioned above are site-specific fungicides, which should only be used when the disease pressure is high (i.e., when there are extended periods of moisture during flowering and preharvest).  Further, when these conditions occur, make sure to tank mix site-specific fungicides with thiram or captan, which will provide some level of disease control in case there is a resistance issue. Based on our results, most of the site-specific fungicides have resistance issues, and only group 7 materials seem to be more reliable at this point, which is a cause for concern since overuse of them could result in the loss of their activity as well.   Fungicide resistance profiles of Botrytis can vary from farm to farm and year to year. As an example, resistance to fludioxonil (in Switch) has seemed to increase significantly in recent years, as it has been commonly used for control of both Botrytis and anthracnose as well as the new disease Neopestalotiopsis.  The information presented here can nonetheless be used as a general guide regarding which products are most likely to have decreased effectiveness, especially if they have been used frequently on your farm or by your plant supplier.

All of this means that cultural controls are more important than ever to maximize foliage drying and minimize periods of wetness. Keep matted rows narrow, remove dead leaves from plasticulture plantings in the spring, and consider increasing plant spacing on plasticulture beds.  Cultivars that are not overly vegetatively vigorous or that are more resistant to fruit rots should be used when possible. Breeding programs that focus more on disease resistance and less on size and appearance may become more valuable over time, as will be the use of high tunnels and low tunnels. Many ongoing studies and field trials are identifying how new and existing biorational fungicides (such as biological materials and essential oils) can be best incorporated into conventional spray programs, serving as another way to reduce the use of site-specific fungicides. Moreover, soft/organic materials may also be more reliable and effective under lower disease pressure, which could be achieved by integrating the above-mentioned practices and disease-tolerant cultivars. Stay tuned for changes to recommendations as further studies proceed.

This work was supported by the AFRI program of the U.S. Department of Agriculture, National Institute of Food and Agriculture, under award No. 2023-67013-39164. The findings and conclusions in this preliminary publication have not been formally disseminated by the U. S. Department of Agriculture and should not be construed to represent any agency determination or policy.