First Report of Rhizoctonia solani AG-2-4 on Carrot in Georgia

Anastomosis group-2-4 (AG-2-4) of Rhizoctonia solani Kühn was formally described in 2002 (1), but it was first collected in 1983 in Georgia from corn (Zea mays L.) exhibiting symptoms of crown and brace root rot. Although occasionally present on diseased corn roots, the role that isolates of AG-2-4 play in crown and brace root rot of corn is not specifically known (2). More recently, as part of a broad multi-year (1996 to 2000) survey of root diseases in field grown carrot (Daucus carota L.), isolates of R. solani AG-2-4 were recovered from diseased carrot plants in various stages of growth from fields of sandy loam soil at many locations in southern Georgia, including commercial fields in Coffee and Tift counties. During the 1996 to 2000 growing seasons, 123 isolates of Rhizoctonia sp. (including multinucleate and binucleate types) were collected from lesions on developing and mature carrot roots. Of these, 34% were AG-2-4, 10% were AG-2-2IV, 6% were AG-4, and 32% were binucleate Rhizoctonia sp. The remaining 18% were lost prior to AG typing. An additional 40 isolates were collected from carrot seedlings or soil and of these, 55% were AG-4, 18% were AG-2-2IV but none were AG-2-4. Virulence on carrot seedlings by two isolates of AG-2-4 (777R1P5-SL2 and 758C) was compared with virulence of isolates of AG-4, AG-2-2IV, AG-2-1, and binucleate Rhizoctonia sp. Carrot seeds soaked for 5 min in 0.5% NaOCl were planted in petri dishes containing moist autoclaved sandy loam soil. Each dish was inoculated in the center with a 10-mm-diameter disk cut from a 9-day-old potato dextrose agar (PDA) culture of the appropriate isolate. Petri dishes were placed in a 26°C incubator for 9 days, and then the seedlings were rated for disease. Virulence on mature carrot root tissue was also assessed on the same set of isolates. Cross sections of carrot roots (5 to 10 mm thick) were surface disinfested in 0.5% NaClO for 5 min. Three cross sections were placed on moist filter paper in sterile petri dishes and each was inoculated with a 5-mm-diameter disk of inoculum cut from 8- to 10-day-old cultures growing on PDA. All treatments were rated for damage following incubation on a lab bench at 21 to 24°C for 7 days. Isolate 777R1P5-SL2 caused moderate damage to seedlings but minimal rotting of mature carrot root tissue. Isolate 758C did no damage to either seedlings or root tissue. The AG-4 and AG-2-2IV isolates killed all seedlings and caused extensive rot on mature root tissue. The AG-2-1 isolate caused moderate damage to seedlings and mature root tissue, whereas isolates of binucleate Rhizoctonia sp. damaged neither seedling nor mature root tissue. These data suggest that some isolates of R. solani AG-2-4 may be capable of causing minor damage to carrot seedlings in the field in Georgia, but isolates of R. solani AG-4 and AG-2-2IV pose greater threat to seedlings and mature roots of carrot. Published data shows that isolates of AG-2-4 can kill seedlings of lettuce, cauliflower, and broccoli in the laboratory (1). R. solani AG-2-4 also may be capable of killing these crops in the field, all of which are grown commercially in Georgia. To our knowledge, this is the first report of R. solani AG-2-4 on carrot in Georgia.