By Navjot Kaur and Bhabesh Dutta
Alternaria brassicicola is a part of the Alternaria complex that causes leaf blight and head rot (ABHR) in brassica crops. In our study, we observed that commercial broccoli seeds can be naturally contaminated with pathogenic and aggressive A. brassicicola. About one-third of the commercial broccoli seedlots screened from two commonly grown cultivars in Georgia were naturally infested with A. brassicicola. Further, we also determined the ability of these isolates (pathogen propagules) to infect broccoli seedlings, and the results indicate that >90% of these isolates were highly aggressive on broccoli leaves (Kaur and Dutta. 2024). Our study suggests that infested seedlots can play an important role in introducing the pathogen in transplant houses and commercial broccoli production fields.
In addition to that, we also provide evidence that A. brassicicola isolates recovered from naturally infested commercial broccoli seedlots have reduced sensitivity to azoxystrobin (quinone-outside inhibitor (QoI)) fungicide, which is frequently used for the management of ABHR in broccoli. Such introductions of pathogen with reduced sensitivity to QoI fungicides may result in reduced efficacy of azoxystrobin against A. brassicicolaunder field conditions. Hence, fungicide programs with different modes of action are recommended in rotation as a risk mitigation strategy for ABHR management.
Furthermore, additional monitoring for natural infestation of broccoli seeds with A. brassicicola combined with assessment for QoIs and SDHI (succinate dehydrogenase inhibitor) fungicides may be needed to better understand the potential risks of fungicide control failures in broccoli. These findings may have implications in considerations for seed-health testing, seed treatments, and greenhouse scouting to limit introduction of infested seedlots in commercial broccoli fields.
Fig. 1 Symptom of Alternaria leaf blight and head rot in broccoli. Panel A displays the necrotic dark brown to black color spots on seedling naturally infected with the pathogen. Panel B displays the necrotic lesions on broccoli heads that can later infect broccoli florets. Panel C indicates heavily infected symptomatic flowering stage of broccoli.
Reference
Kaur, N., and Dutta, B. 2024. Aggressive Alternaria brassicicola with reduced fungicide sensitivity can be associated with naturally infested broccoli seeds. Plant Dis. 108:2154-2161.
Acknowledgment
This project was funded by USDA NIFA SCRI (2020-51181-32062).