The 2012 ASHS Annual Conference

Thielaviopsis basicola is a soil-borne, plant-pathogenic fungus devastating many bedding plants and floricultural crops. Plants infected by this fungus are characterized by stunted shoot growth, yellow or necrotic leaves, and black regions on roots, hence the common name “black root rot”. Despite the significance of black root rot disease, little is known about how the disease is influenced by plant hormones and nutrition. In preliminary experiments, we observed chlorotic and necrotic leaves developing from the lower stem of petunia infected by Thielaviopsis basicola. The absence of typical root symptoms prompted us to question whether the severity of shoot disease symptoms is directly related to the extent of root infection. Therefore, this study was conducted to elucidate how disease development symptoms are influenced by the interaction of phosphorus nutrition and ethylene sensitivity, and whether a correlation exists between the degree of disease symptom development in the shoot and the fungal population in infected roots. Low phosphorus dramatically enhanced disease symptom development, as measured by percent necrotic leaves, in both wild-type and etr1-1 petunias during a 7 week period after transplanting. Ethylene insensitive etr1-1 petunia developed earlier and more severe disease symptoms than the wild-type during the first 4 weeks after transplanting, but showed less severe symptoms at 7 weeks as compared to the wild-type, indicating that ethylene plays an important role in symptom development. Disease symptom development was positively correlated with the number of colonies that grew from root segments plated on a Thielaviopsis selective medium (TB-CEN) in both wild-type and etr1-1 petunia. The etr1-1 petunias displayed more severe disease development symptoms compared to wild-type when the roots were equally infected by T. basicola. Inoculation methods and environmental conditions considerably influenced disease severity.