Species identification and fungicide sensitivity of fungi causing Alternaria Leaf Blight and Head Rot in cole crops in the Eastern U.S.

Abstract

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 broccoli-producing states in the U.S., there is a loss of disease control through the use of QoI fungicides. To understand why there was a reduced sensitivity to QoI fungicides in these states, we isolated Alternaria species 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 multi-locus sequencing and phylogenetic analysis, we identified two species, A. 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, while the EC50 of A. japonica ranged from 7.9 to 27.1 ppm. None of the known target-site mutations that confer resistance to QoI fungicides were identified during screening of either species. Alternaria japonica was first reported on the East Coast of the U.S. in 2020 in South Carolina. The substantially higher EC50 value suggests that its emergence in the Southeastern U.S. 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.