Seung-Hun Yu

Gray Leaf Spot in Peppers Caused by Stemphylium solani and S. lycopersici

A disease causing severe leaf spots in pepper plants has been observed in northern Gyeongbuk and Gangwon provinces in Korea since 1994. The current study diagnosed the disease as gray leaf spot caused by Stemphylium solani Weber and S. lycopersici (Enjoji) Yamamoto, both of which are pathogenic in pepper and tomato plants. Although the disease has been found in almost all areas where peppers are grown, it is more severe in mountain terrains where the nights are cool. Both species of pathogenic fungi were found to sporu-late profusely on V-8 juice agar in plastic or Pyrex glass Petri dishes, although not in domestically-produced glass Petri dishes, when cultured at under irradi-ation from a daylight fluorescent lamp with a 12-hour light and dark alternation. The domestically-produced glass Petri dishes, which are made of window glass, were found to block near ultraviolet wavelengths, around and below 300 nm, which explained why the fungi did not sporulate. However, sporulation decreased at above and most isolates failed to sporulate above . The worst level of disease was obtained when the inoculated plants were incubated with a night and day temperature regime relative to 4 night/day temperature combinations (15/20, 20/25, 25/30, and 30/35).

Analysis of Genetic Relatedness in Alternaria species Producing Host Specific Toxins by PCR Polymorphism

Twenty universal rice primers (URPs) were used to detect PCR polymorphisms in 25 isolates of six different Alternaria species producing host specific toxins (HST). Eight URPs could be used to reveal PCR polymorphisms of Alternaria isolates at the intra- and inter-species levels. Specific URP-PCR polymorphic bands that are different from those of the other Alternaria spp. were observed on A. gaisen and A. longipes isolates. Unweighted pair-group method with arithmetic mean (UPGMA) cluster analysis using 94 URP polymorphic bands revealed three clustered groups (A. gaisen group, A. mati complex group, and A. logipes group).

Corky Root of Tomato Caused by Pyrenochaeta lycopersici in Korea

Corky root symptoms caused by Pyrenochaeta lycopersici were observed on the roots and stem base of tomato plants in Korea. Symptoms on infected plants typically appeared as stunting and generally lacking vigor, and infected plants die back from the foliage tips after fruits have set. Brown lesions appearing with bands around the roots were characteristic symptoms of the disease. The lesions become swollen and cracked along the length of the root with corky appearance. Based on cultural and morphological characteristics, the fungus from the diseased plants was identified as Pyrenochaeta lycopersici. Pycnidia were solitary, globose to subglobose, brown to black, darker around the neck region, and measured 173-215 in diameter with septate setae up to 102-1326.5 . Conidia were hyaline, unicellular, and 4.2-4.7l.5-2.0 long. Optimum temperature for mycelial growth of the p. lycopersici isolates ranged from to . Fifteen isolates off lycopersici were tested for pathogenicity to susceptible and tolerant cultivars of tomato plants by artificial inoculation. Three isolates of P. lycopersici induced typical corky root discoloration on susceptible tomato cultivars but not on tolerant tomato. This is the Erst report in Korea of tomato corky root disease caused by P. lycopersici.

Verticillium Wilt of Potato Caused by Verticillium albo-atrum in Daegwallyong Area in Korea

Verticillium wilt was first observed in 2001 on potatoes (Solanum tuberosum) cv. Superior at Daegwallyong area, one of the major seed potato producing areas in Korea. The wilted potato plants showed typical symptoms including gradual yellowing and interveinal necrosis. There was discoloration in the vascular tissues of the infected stems which turned light brown. Fungal isolates from discolored vascular tissues were whitish to creamy with folding on potato dextrose agar medium, where they used to produce resting dark mycelia but no micro-sclerotia. Conidiophores were septate with side branches, swelled at the base, and arranged in a whorl. Conidia were 2.5-11.22.0-4.5 um in size and were borne in small clusters at the tips of phialides. Optimal temperature range for mycelial growth was . Based on these cultural and morphological characteristics, the fungus was identified as Verticillium albo-atrum Reink & Berth. Pathogenicity tests by root dipping method revealed that the fungus caused the same symptoms as observed in naturally infected potato plants. This is the first report of Verticillium wilt on potato caused by Verticillium albo-atrum in Korea.

Wilt of Perilla Caused by Fusarium spp.

A survey of Fusarium wilt of perilla was conducted in 12 locations in Korea from 1999 to 2001. The disease occurred in 74 out of 187 fields in the 12 locations surveyed, and incidence of the disease reached up to 30% at its maximum in some perilla fields in Seosan and Dangjin. Incidence of the disease in the other locations ranged from 0.2 to 20%. A total of 327 isolates of Fusarium spp. were obtained from stems and roots of the diseased perilla plants. The isolates were identified based on their morphological characteristics. Out of the 327 isolates of Fusarium, 277 isolates from 12 locations were identified as F. oxysporum, 11 isolates from three locations as F. solani,17 isolates from two locations as F. equiseti, 4 isolates from one location as F. avenaceum and 6 isolates from one location as F. subglutinans. The other 12 isolates of Fusarium from four locations were unidentified. Twelve isolates of F. oxysporum and two isolates each of the other Fusarium spp. were tested for their pathogenicity to five cultivars of perilla. Seven isolates of F. oxysporum were strongly pathogenic to some perilla cultivars, but the other five isolates were weakly or not pathogenic. One isolate of F. solani was strongly pathogenic to all the perilla cultivars tested, but another isolate was not pathogenic. All the isolates of F. equiseti, F. avenaceum, and F. Subglutinans tested were not pathogenic to any of the perilla cultivars tested. Symptoms on the perilla plants induced by artificial inoculation with strongly pathogenic isolates of F. oxysporum and F. solani appeared as wilt, stem blight, and root yet, which were similar to those observed in the fields. The isolates which induced symptoms by artificial inoculation were re-isolated from the lesions of the perilla plants inoculated. All the isolates of F. oxysporum tested were not pathogenic to eight other crops inoculated. Results of this study reveal that F. oxysporum is the main pathogen of perilla wilt and that it is host specific to perilla. forma specialis of F. oxysporum causing wilt of perilla is proposed as perillae.