Fungicide Sensitivity of Sclerotinia sclerotiorum from U.S. Soybean and Dry Bean, Compared to Different Regions and Climates.

Fungicide use is integral to reduce yield loss from Sclerotinia sclerotiorum on dry bean and soybean. Increasing fungicide use against this fungus may lead to resistance to the most common fungicides. Resistance has been reported in Brazil (Glycine max) and China (Brassica napus subsp. napus), however, few studies have investigated fungicide sensitivity of S. sclerotiorum in the United States. This work was conducted to determine if there was a difference in fungicide sensitivity of S. sclerotiorum isolates in the United States from: (i) dry bean versus soybean and (ii) fields with different frequencies of fungicide application. We further hypothesized that isolates with fungicide applications of a single active ingredient from tropical Brazil and subtropical Mexico were less sensitive than temperate U.S. isolates due to different management practices and climates. The EC50(D) fungicide sensitivity of 512 S. sclerotiorum isolates from the United States (443), Brazil (36), and Mexico (33) was determined using a discriminatory concentration (DC) previously identified for tetraconazole (2.0 ppm; EC50(D) range of 0.197 to 2.27 ppm), boscalid (0.2; 0.042 to 0.222), picoxystrobin (0.01; 0.006 to 0.027), and thiophanate-methyl, which had a qualitative DC of 10 ppm. Among the 10 least sensitive isolates to boscalid and picoxystrobin, 2 presented mutations known to confer resistance in the SdhB (qualitative) and SdhC (quantitative) genes; however, no strong resistance was found. This study established novel DCs that can be used for further resistance monitoring and baseline sensitivity of S. sclerotiorum to tetraconazole worldwide plus baseline sensitivity to boscalid in the United States.

Species Identification and Fungicide Sensitivity of Fungi Causing Alternaria Leaf Blight and Head Rot in Cole Crops in the Eastern United States.

Alternaria leaf blight and head rot is an important disease of broccoli and other cole crops. With no resistant host varieties, fungicides are utilized to manage this disease. However, anecdotal evidence suggests that, in southeastern U.S. broccoli-producing states, there is a loss of disease control through the use of quinone outside inhibitor (QoI) fungicides. To understand why there is a reduced sensitivity to QoI fungicides in these states, we isolated Alternaria spp. from symptomatic lesions on cole crops from Georgia and Virginia (two states with observations of loss of fungicide sensitivity) as well as New York (a state with no observations of loss of fungicide sensitivity). Using multilocus sequencing and phylogenetic analysis, we identified two species, Alternaria brassicicola and A. japonica. Whereas A. brassicicola was isolated in all states, A. japonica was only isolated in Georgia. Next, we wanted to determine the sensitivity of these isolates to azoxystrobin-an active ingredient in some QoI fungicides-by estimating the effective concentration at which only 50% of spores germinate (EC50). The EC50 of A. brassicicola ranged from 0.01 to 0.17 ppm, whereas that of A. japonica was 8.1 to 28.1 ppm. None of the known target-site mutations that confer resistance to QoI fungicides were identified during screening of either species. A. japonica was first reported on the east coast of the United States in 2020 in South Carolina. The substantially higher EC50 value suggests that its emergence in the southeastern United States may play at least a part in the observed loss of disease control. However, further in planta and field studies are needed to thoroughly test this hypothesis.