Agricultural and Forestry Experiment Station, University of Alaska, Fairbanks, Fairbanks, Alaska 99775-7200, USA

Background and objectives
Sclerotinia sclerotiorum causes stem rot on many garden flowers in high latitude regions. This disease is especially devastating on petunia (Petunia spp.), one of the most popular garden flowers in Alaska. The pathogen produces many large sclerotia on the surface and in the hollowed center of the stem. The sclerotia survive the harsh winters well; they can stay viable in the soil many years. Each spring, these sclerotia serve as the primary inoculum of the disease. In many cases, this disease is so severe that it makes the cultivation of petunias impossible. All of the popular cultivars are extremely susceptible to the disease. Control of this disease is very difficult. Disease control using chemicals is costly, often ineffective and incites great concern in northern countries because of the fragility of the environment. An environmentally benign, effective method of control is much needed. Trichoderma atroviride is a versatile, aggressive hyperparasite selected from more than 54 fungal and 400 bacterial hyperparasites isolated from the sub-arctic region of Alaska. It can parasitize a wide spectrum of pathogenic fungi. A temperature range of 4-33C and a pH range of 1.5 to 10.0 makes it especially useful in suppressing pathogens flourishing under these conditions. It is rhizosphere competent and has plant growth promotion abilities. The main objective of this project is to evaluate the effects of T. atroviride on S. sclerotiorum under laboratory, greenhouse and field conditions.

Results and conclusions
Results of dual cultures and other laboratory tests indicated that T. atroviride is an effective hyperparasite of S. sclerotiorum. The hyphae of T. atroviride penetrate the hyphae of S. sclerotiorum and cause coagulation of cytoplasm and cell lysis. At the macroscopic level, the expansion of the S. sclerotiorum colony was arrested upon contact with the mycelia of T. atroviride, and as the hyperparasitism progressed, the entire S. sclerotiorum colony was destroyed. T. atroviride is found to be able to control sclerotinia stem rot on petunia under greenhouse conditions. Except for plants in the Topsin M treatment, T. atroviride was found on and in the tissues of roots and stems of petunia plants in all treatments, including the blank control and S. sclerotiorum control. Finding T. atroviride present inside the petunia stem tissues suggests that T. atrovirde is capable of providing more than topical protection against S. sclerotiorum. This result explains why no diseased petunia plants were found in any of the treatments. Plants remained healthy in all of the trials conducted, including S. sclerotiorum control. Results of the field trial conducted using a split-block experimental plot design indicated that petunia plants treated with Topsin M and T. atroviride stayed in bloom longer. The Topsin M was applied as a drench twice. T. atroviride was applied to the seedlings and/or into the soil at the time of transplanting. These plants were also significantly healthier than the blank control, as measured by fresh weight and dry weight. Although the fresh weights of petunia treated with Topsin M and T. atroviride double treatments (seedling and soils) were significantly greater than T. atroviride applied to soils alone, no significant differences were found. Application of Topsin M and T. atroviride to seedlings and field soils yielded significantly fewer sclerotia per plant. Results of the findings under laboratory, greenhouse and field conditions strongly suggest that T. atroviride is an aggressive, effective biological control agent of S. sclerotiorum.

1. Pirone PP, 1978. Diseases and pests of ornamental plants. 5th edn. Wiley, Chichester.