A recently published paper in Phytopathology by Naegele et al. adds to the work done by Cosseboom (MS Cal Poly, 2018) with regards to fungicide resistance in Botrytis cinerea in California. Cosseboom’s study was conducted in 2016 and included 888 isolates collected from strawberries throughout the three major growing districts in the state. This new study was done in 2018 and included B. cinerea isolates collected from blueberry (n = 344), strawberry (n = 302), and grape (n = 312) in six California counties (San Joaquin, Fresno, Tulare, Kern, Monterey and Santa Cruz). Presumably, all the strawberry isolates came from Monterey and Santa Cruz counties. Fungicide resistance screening against eight active ingredients was done on a subsample of 431 isolates: blueberry (n = 145), strawberry (n = 140), and grape (n = 146).  

The fungicides evaluated and a comparison of the results is presented in Table 1. Nearly 90% of the 140 strawberry isolates were resistant to one or more of the fungicides evaluated.

Comparing results between this study and Cosseboom’s, we see some noteworthy differences. I’m playing a bit loose with this data since Naegle’s numbers are from three crops whereas Cosseboom only looked at strawberry. Still, I think there’s value in the comparison. On the positive front, resistance to Elevate was lower in Naegele’s study and Switch remains the same, with fludioxonil (one of the components in Switch) having the lowest resistance frequency of any active ingredient. In contrast, resistance has increased to Rovral, Pristine and Luna Sensation. In the case of Luna Sensation, this may be due to its relatively recent registration for use in strawberries and its dual role against Botrytis fruit rot and powdery mildew, thus increasing its use and resulting selection pressure. The rise in resistance to Rovral might be due to its use in blueberries and grapes since this fungicide is not currently used in strawberries.

Table 1. Frequency of resistance in 431 isolates of Botrytis cinerea collected in 2018 from blueberry, strawberry and grape to eight fungicide active ingredients, compared to Cosseboom’s results in 2016 from strawberry.

		Resistance frequency (%)
Active ingredient	Trade name	Naegele et al. results – 2018	Cosseboom results – 2016 (late season)
cyprodinil	one of the components in Switch	50	50
iprodione	Rovral	59	10
fenhexamid	Elevate	47	90
fludioxonil	one of the components in Switch	2	5
thiabendazole	not used in strawberries but similar mode of action to thiophanate-methyl	64	97
pyraclostrobin	one of the components in Pristine and Merivon	83	98
boscalid	one of the components in Pristine	88	35
fluopyram	one of the components in Luna Sensation	52	7

Another interesting finding for California strawberries is that resistance to multiple fungicides in a single fungal isolate is very high with 68% of the 140 isolates collected from strawberries having resistance to six or seven active ingredients (Fig. 2). In Cosseboom’s study, we found a similar level of resistance but for three or four active ingredients instead of six or seven. That’s a big jump in multiple resistance.

While this study used a smaller sample size for strawberries than Cosseboom’s, it points to some trends that should be concerning to California strawberry growers, PCAs and fungicide registrants. Resistance is increasing to most active ingredients and multiple resistance is also increasing. This is further compromising fungicide efficacy, leading to reduced efficacy and increased losses due to Botrytis fruit rot.

References

Cosseboom, S.D., Ivors, K.L., Schnabel, G, Bryson, P.K. and Holmes, G.J. 2019. Within-season shift in fungicide resistance profiles of Botrytis cinerea in California strawberry fields. Plant Disease 103:59-64. https://doi.org/10.1094/PDIS-03-18-0406-RE

Naegele, R.P., Abdelsamad, N., DeLong, J.A., Saito, S., Xiao, C-L and Miles, T.D. 2022. Fungicide resistance and host influence on population structure in Botrytis spp. from specialty crops in California. Phytopathology 112:2549-2559. https://doi.org/10.1094/PHYTO-03-22-0070-R