Use strobilurin fungicides wisely to avoid fungicide resistance development

Strobilurins are fungicides that are modeled after an antifungal substance produced by a small forest mushroom called Strobilurus tenacellus. This mushroom grows on pine cones and uses an antifungal substance to suppress other fungi which may be competing for the same food source. Synthetic strobilurins were made to be more resistant to UV light degradation than the natural chemical produced by S. tenacellus. All strobilurins have the same mode of action, i.e., they inhibit the electron transfer in mitochondria, disrupting respiration and thereby causing the fungus to run out of energy and die. Strobilurins belong to the group of QoI’s (quinone outside inhibitors) based on the specific site that they inhibit. They include azoxystrobin (Abound), kresoxim-methyl (Sovran), pyraclostrobin (Cabrio) and trifloxystrobin (Flint). The fungicide Pristine is a mixture of pyraclostrobin and boscalid. Boscalid is not a strobilurin but belongs to the carboxamide chemical class. Interestingly, some of the strobilurins also have phytotoxicity to certain plant species; e.g., Abound is phytotoxic to apples and Sovran is phytotoxic to certain sweet cherry varieties. Pristine and Flint are phytotoxic to Concord grapes. Caution must be taken when applying these products in the vicinity of sensitive crops.

Since their first EPA registration in 1997, strobilurins have become valuable tools for managing diseases in numerous crops, including grapes and berry crops, because of their systemic nature and broad spectrum of activity against different groups of plant pathogens. In berries, they are especially effective against fruit rot and foliar fungi. In grapes, they provide broad-spectrum control against powdery mildew, downy mildew, Phomopsis cane and leaf spot and black rot. They are not very strong against Botrytis, however. Strobilurins become rainfast quickly and have translaminar activity, which means that they can move from one side of a leaf to the other, providing disease control on both leaf surfaces. Strobilurins have an outstanding ability to inhibit spore germination, thus they should be most useful early in disease development. They do not have much post-infection activity. Some strobilurins (e.g., Abound and Flint) are listed as “reduced-risk” by the EPA, which means that they have relatively low mammalian toxicity. However, they are toxic to fish and other aquatic organisms, so regulations must be followed for use around bodies of water.

Since strobilurins have a site-specific mode of action, they are prone to fungicide resistance development in target fungi because a single mutation in a fungus can block their action. Where strobilurin resistance has occurred, the pathogen strains have exhibited a high level of resistance that cannot be overcome by increasing the fungicide application rate. Continued and exclusive use of strobilurin fungicides may allow resistant strains to build up over time and may lead to control failure and loss of the fungicide as a disease management tool. Strobilurin resistance has been reported in grapevine powdery and downy mildew in various eastern US states and has not also been confirmed in grapevine powdery mildew in Michigan. Strobilurin resistance has been reported in Colletotrichum acutatum in strawberries in Florida as well.

The goal is not to manage resistance once it has developed, but rather to prevent or delay the development of fungicide resistance in the first place. To do this, it is important to limit the number of strobilurin applications, for instance by alternating them with fungicides with a different mode of action. A good guideline is for strobilurins to make up no more than 1/3 of all fungicide applications during the growing season. In addition, regular disease scouting to determine the actual need for fungicide sprays and non-chemical management practices, such as sanitation, canopy management, and biological control are also important. Do not use more than four total applications and two sequential applications of any strobilurin fungicide per season.

To extend the useful life of strobilurin fungicides as a group, the fungicide labels limit the number of applications (total and sequential) per season (Table 1). This also includes pre-mixes such as Adament which have a strobilurin component. It is advisable to alternate strobilurins with block treatments (2 to 3 sprays) of fungicides with a different mode of action, including multi-site contact materials that have a low risk of resistance development, such as Bravo, Captan, and Ziram, or biological control agents, such as Serenade. Tank-mixing strobilurins with other fungicides is usually not necessary or cost-effective since they already have a broad spectrum of activity. However, in grape vineyards with known or suspected resistant powdery mildew strains, tank-mixing with sulfur or Ziram may aid in control. In fact, in that case, it would be better to avoid strobilurins altogether or to limit them to one application during a period with lower disease pressure.

Table 1. Summary of label information for strobilurin fungicides for small fruit crops, 2010.

Product
Active ingredient
Recommended application rate per acre
Max. rate or number of applications per acre per season
Max. number of sequential applications
REI (hours)
PHI (days)
Abound F
azoxystrobin (22.9%)
10.0 – 15.5 fl oz (grapes)
 
6.2 – 15.4 fl oz (raspberries, blackberries)
 
6.2 – 15.4 fl oz (blueberries)
 
6.2 – 15.4 fl oz
(cranberries)
 
6.2 – 15.4 fl oz
(strawberries)
 
92.3 fl oz
 
 
92.3 fl oz
 
 
 
46 fl oz
 
 
92.3 fl oz
 
 
61.5 fl oz
 
2
 
4 h
 
 
14 d
(grapes)
 
0 d
(raspberries, blackberries)
 
0 d (blueberries)
 
3 d (cranberries)
 
0 d (strawberries)
 
Cabrio EG
pyraclostrobin (20%)
 
14 oz  (blueberries, raspberries)
 
12-14 oz (strawberries)
56 oz (blueberries, raspberries)
 
70 oz (strawberries)
 
2
12 h
0 d
Flint
trifloxystrobin (50%)
 
 
1.5 – 4 oz (grapes)
 
2 - 3.2 oz (strawberries)
24 oz
(grapes)
 
19.2 oz (strawberries)
 
Max. 6 applications
 
2
 
 
12 h
14 d
(grapes)
 
0 d
(strawberrries)
Pristine
pyraclostrobin (12.8%) + boscalid (25.2%)
8 – 10.5 oz; except 18.5-23 oz for Botrytis) (grapes)
 
 
18.5 – 23 oz (blueberries, raspberries)
 
 
18.5 – 23 oz (strawberries)
 
69 oz (3 applications for Botrytis; 5 for other diseases) (grapes)
 
92 oz
(4 applications) (blueberries, raspberries)
 
115 oz (5 applications)
(strawberries)
 
2
12 h
 
5 d for grape cane typing
 
14 d (grapes)
 
0 d (berries)
 
Sovran
kresoxim methyl (50%)
 
3.2 – 6.4 oz
4 applications (wine and table grapes)
 
3 applications (other grapes)
 
2
12 h
14 d (grapes)

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