Resistant fungus detected in broccoli seeds in the US

A study by researchers at the University of Georgia revealed that the mushroom Alternaria brassicicola, present in commercial broccoli seeds in the United States, has developed cross-resistance to two fungicides from the succinate dehydrogenase inhibitor (SDHI) group: boscalid and penthiopyrad.

The researchers analyzed 58 isolates of A. brassicicola obtained from naturally infested seeds of two broccoli cultivars. The study used radial growth tests in culture media with different concentrations of the fungicides boscalid, penthiopyrad, and fluopyram.

Of the 58 isolates tested, 93% were resistant to boscalid and penthiopyrad. Most grew even at the highest concentrations of both products. In contrast, all isolates demonstrated sensitivity to fluopyram.

Cross-resistance

Approximately 15% of the isolates tested showed resistance to both SDHI fungicides and azoxystrobin, a quinone external inhibitor (QoI) fungicide. These cases indicate a cross-resistance phenomenon that can severely reduce chemical disease management options.

Genetic analysis revealed a point mutation in the sdhC gene, which encodes one of the subunits of the succinate dehydrogenase enzyme. This mutation, identified as H134R, appeared exclusively in isolates resistant to the fungicides boscalid and penthiopyrad.

Based on this mutation, researchers developed a molecular PCR test with specific primers to rapidly detect resistant isolates in seed lots.

Agricultural management

the fungus A. brassicicola causes Alternaria leaf spot and head rot in broccoli. The disease affects leaves and inflorescences, causing significant losses in yield and quality. The use of SDHI and QoI fungicides has been one of the main control tools.

The detection of resistant isolates in commercial seeds suggests that the problem may spread through the seedling production chain and reach regions where the use of these fungicides is still limited. This compromises the effectiveness of management programs and may lead to unexpected failures in disease control.

Resistant isolates

The researchers evaluated mycelial growth and spore germination of isolates with different sensitivity profiles.

Boscalid-resistant isolates showed a lower mycelial growth rate compared to sensitive isolates. However, there was no significant difference in spore germination.

This indicates that resistant isolates remain competitive under natural conditions and can spread easily.

Conserved genetic structure

The team also conducted phylogenetic and structural analyses of the sdhB, sdhC, and sdhD genes. The results showed similarity between A. brassicicola and other species of the genus Alternariaas the A.solani e A. alternata.

The conservation of coding regions suggests that point mutations such as H134R can arise and easily become fixed among populations of different species, favoring the spread of resistance.

Further information at doi.org/10.1128/aem.01083-25