Identification of the causal fungal agents of potato seed tubers dry rot and evaluation of the effect of some disease severity factors on the disease progress

Fusarium dry rot is one of potatoes' most significant postharvest fungal diseases, reducing crop yield and seed tuber quality. Utilizing certified seed tubers treated with chemical fungicides is the simplest and most cost-effective method for preventing dry rot disease damage to potatoes. Monitoring for diseases transmitted by tubers enhances the quality of seed tubers produced and reduces the risk of disease establishment and soil contamination. This study aimed to identify the causal fungal agents of potato seed tubers dry rot, determine the susceptibility of seed samples from potato seed farms to Fusarium dry rot agents, as well as to evaluate some of the effective factors in disease severity, particularly the levels of extracellular enzyme activity and fusaric acid production on the disease index and dry rot volume of potato tubers.

To identify Fusarium species responsible for dry rot disease in the seed tubers of Agria, Banba, Jelly, Challenger, Ramus, Sagitta, Sante, Sifra, Fabula, and Colomba cultivars of potato grown in Ardabil (Ardabil), Chaharmahal & Bakhtiari (Borujen), Razavi Khorasan (Fariman and Torbat-e Heydarieh), Zanjan (Khodabandeh), Fars (Abadeh and Eqlid), Lorestan (Khorramabad), Markazi (Arak) and Hamedan (Razan, Kabudarahang, and Bahar) provinces were sampled according to the standard guidelines of the National Seedling Health and Propagation Materials Laboratory. The pathogenicity test was carried out on artificially inoculated potato tubers tubers cv. Agria for determining the volume of dry rot caused by Fusarium isolates. Furthermore, spectrophotometry and high-performance liquid chromatography (HPLC) methods were used to evaluate in vitro activities of amylase and cellulase, as well as fusaric acid production.

The results indicated that approximately 18% of the seed tubers samples collected from different fields were infected with Fusarium dry rot disease in the 1 to 3% range. A total of 26 isolates were identified based on morphological and molecular characteristics belonging to F. solani sensu lato (13 isolates, 11%), F. oxysporum (8 isolates, 7%), and F. sambucinum (5 isolates, 5%). Analyzing the activity of extracellular enzymes revealed that all recovered Fusarium isolates were capable of producing amylase and cellulase enzymes but at varying levels. The maximum amount of extracellular enzymes produced by Fusarium spp. isolates ranged from 286 to 675 μg mL-1 for cellulase and from 103.5 to 317 μg mL-1 for amylase. According to the results, cellulase peaked sooner than amylase, but both enzymes contributed to the development of dry rot in potato tubers. The results revealed that approximately 38% of the isolates produced fusaric acid in the 13-32 μg kg-1 range. Moreover, the disease index level and the volume of dry rot caused by different Fusarium species isolates on potato tubers varied.

It appears that the amount of dry rot caused by Fusarium spp. isolates on potato tubers are affected by the production of fusaric acid and the activity levels of cellulose and amylase. This research provides new insights into the health status of potato seed tubers, the predominant Fusarium species causing dry rot disease, and the effect of the level of extracellular enzymes and fusaric acid secreted by fungal isolates on the disease progress in potato tubers, which can be applied to the revision of the national standard for seed tuber health and effective disease management during storage. The dry rot disease and its detrimental impact on tubers significantly threaten the official seed tubers certification system, production, and storage procedures. The current study is the first report on identifying the Fusarium dry rot disease agents from potato seed tubers in Iran and investigating the relationship between the disease severity of isolates and their extracellular enzyme activity, and fusaric acid production.