Biocontrol of fungal soilborne pathogens by Pythium oligandrum

#Within the fungal genus Pythium are plant pathogenic species which can cause significant losses in vegetable production. The value of vegetable crops grown in the state of Florida totaled over $1.79 billion for the 1991-92 crop season with much of the cabbage, cucumber, pepper, and tomato crops planted as transplants. Tomato alone accounts for $735 million in this production total and over 80% of the crop is transplanted. The use of vegetable transplants in commercial field production systems is important in many areas of the United States. In California, all of the celery, fresh market tomato and pepper, and most of the cauliflower and broccoli are grown as transplant crops. In Monterey County alone, the value of the transplants grown in 1992 amounted to nearly $18 million and represented a final crop value of $176 million.

#Plant pathogenic Pythium species can kill a plant at the seedling stage or can reduce crop yield by destroying the root system of a mature plant. While diseases in the seedling stage are often controlled by fungicide application, the continued use of certain highly effective fingicides, e.g., metalaxyl, has faced regulatory uncertainty for use in vegetable transplant greenhouse production systems. In addition, continued use of a particular fungicide can result in the development of tolerance by the pathogen. Fumigants such as methyl bromide, which are routinely used on high cash-value crops, also face regulatory uncertainty. Thus, disease control (in particular damping-off) in the production greenhouses as well as in the field following transplanting are a major concern.

#In addition to the plant pathogenic species of Pythium, some members of this genus exist strictly as soil saprophytes (Van der Plaats-Niterink, 1981, Monograph of the genus Pythium. Studies in Mycology No. 21. Centraalbureau Voor Schimmelcultures, Baarn, The Netherlands), and several have been identified that are pathogenic on mammals (de Cock et al., 1987, J. Clin. Micro. 25:344-349), fish (Van der Plaats-Niterink, 1981), and insects (Saunders et al, 1988, J. Invert. Path. 52:360-363). One species, Pythium oligandrum Drechsler, is not pathogenic on plants and is effective in protecting plants from attack by pathogenic species.

#FIG. 1 shows linear growth rates of isolates of Pythium oligandrum on corn meal agar for a 24hour period when incubated at the indicated temperatures .

#The subject invention relates to a novel, efficacious biocontrol agent comprising a nonpathogenic species, Pythium oligandrum. The subject isolate is not pathogenic on plants and shares a similar ecology to the pathogenic species within the same Pythium genus. Therefore, the subject isolates have the advantage of being active under similar environmental conditions and compete for occupation of the same ecological niche as the pathogens. A number of isolates have been evaluated for efficacy in controlling damping-off of tomato transplants. Several isolates have been identified that provided consistent protection from disease in field evaluations over a five-year period.

#The P. oligandrum of the subject invention are different isolates than the previously-described Pythium species used as biocontrol agents, (e.g., Polygandron described in the aforementioned '317 patent). Another important difference is the different process, e.g., culture medium used to grow the biochemical agent. There is a great deal of variability among isolates, e.g., cultural characteristics (growth rates, response to temperature, spore formation, germination), as well as efficacy in protecting from disease.

#Morphological characteristics of the isolates conform to the taxonomic characteristics described for the species (Van der Plaats-Niterink, 1981). For isolates grown on liquid grass blade cultures, sporangia are contiguous, forming irregular aggregates consisting of subglobous elements with connecting filamentous parts. Mostly formed intercalary but occasionally terminal. Zoospores are produced. Oogonia average 25 .mu.m in diameter and are covered with conical protuberances 5-7 .mu.m long and 2-3 .mu.m at the base. Antheridia are generally lacking. Oospores are aplerotic and average 22 .mu.m in diameter. Cultural characteristics such as linear growth rates and sporulation may be found in FIG. 1 and Table 1, respectively. Details of molecular characterization of isolates as P. oligandrum may be found in Example 6.