First Report of Seedling Blight of Eastern Poison Ivy (Toxicodendron radicans) by Colletotrichum fioriniae in Virginia

Abstract

Colletotrichum fioriniae is a member of the large cosmopolitan C. acutatum species complex (2). Known agricultural hosts of C. acutatum include apple, European blueberry, grape, olive, papaya, and strawberry (2). In contrast, the life history of C. fioriniae ranges from an epizootic of certain scale insect populations to an endophyte of plants (3,4). The present study extends the phytopathology of C. fioriniae to include poison ivy seedlings. Poison ivy (Toxicodendron radicans) drupes were collected from solitary lianas in Roanoke and Montgomery counties, Virginia. These drupes were subjected to experiments aimed at producing sterile seedlings (1); however, there was extensive blighting and wilting in the germinated seedlings. Associated with the drupes and seedlings was a fungus with white to pale olivaceous grey mycelium with orange blister-like conidiomata and sclerotial masses enclosing the drupe mesocarp as well as conidiomata emerging from blighted, necrotic leaves. Condiomata were plated onto acidified potato dextrose agar (APDA) and oatmeal agar (OA). This consistently yielded colonies identical to those described from diseased tissues and were putatively identified as C. acutatum based on the presence of acervuli containing hyaline, smooth-walled, aseptate conidia with acute ends, the absence of setae, and formation of red pigments in culture (2). Conidial dimensions of four isolates most closely aligned with reported measurements for C. fioriniae (4): mean length ± SD × width ± SD = 15.1 ± 1.7 × 4.9 ± 0.3 μm, L/W ratio = 3.04 on OA. Fungal DNA was isolated and used as template in PCR reactions using oligonucleotide primer pairs corresponding to the internal transcribed spacer (ITS) region, and a portion of the glyceraldehyde-3-phosphate dehydrogenase (GAPDH) genes. The resulting PCR fragments were sequenced and used as queries in BLASTN searches of the GenBank NR database. All of the amplified ITS DNA sequences (497 bp KF944356 and KF944357) were identical to Glomerella/Colletotrichum fioriniae (JN121190 and KF278459). Similarly, the amplified (672 bp) GAPDH sequences (KF944354 and KF944355) were 99.6% similar over the 254 bp overlapping with C. fioriniae (JQ948622). Pathogenicity of two randomly chosen C. fioriniae isolates, TR-123 and TR-126, was confirmed by placing 4.75 mm diam. inoculated agar plugs from 8-day-old fungal cultures or a sterile plug (negative control) at the base of an axenic young seedling ~1.5 to 6.5 cm in height with at least one set of true leaves (1). Each treatment was replicated five times. Acute wilt and blighting of leaves and production of orange acervuli on cotyledons disease symptoms developed by 3 weeks post inoculation (WPI). By 7 WPI all but one of the Colletotrichum-inoculated plants were dead, whereas all of the control plants were healthy with significantly lower area under the disease progress curve values. Colletotrichum was consistently re-isolated, and confirmed morphologically and molecularly, from six of seven diseased seedlings, whereas two of two randomly chosen control seedlings remained asymptomatic and did not yield Colletotrichum. In summary, C. fioriniae may represent a natural biocontrol agent against poison ivy and scale insect herbivores thereof.