with differences in the length and branching of the aliphatic amino acid residue. When tested against yeast, Saccharomyces cerevisiae, mycosubtilin was most active (LC50 = 10 µg/mL), followed by iturin-A and bacillomycin L (both 30 µg/mL) (Besson et al. 1979). Iturin-C is inactive against fungi, including Penicillium chrysogenum (Peypoux et al. 1978).
Researchers at USDA ARS have investigated the structure-activity relationship of the iturins by synthesizing a number of analogues differing in amino acid chain length. They reported that the activity of the iturins increased with the length of the fatty acid side chain and the terminal branching in the order iso > normal > ante-iso (Bland et al. 1995) and that the “amounts of iturins obtained from natural production are inadequate to be commercially viable” on the basis of their work with a number of iturin-producing strains of Bacillus. However, only a small number of potential Bacillus strains have been screened for fungicidal activity. AgraQuest is developing a naturally occurring Bacillus subtilis, which has shown excellent activity for some foliar and fruit diseases. Investigators isolated it in a screening program directed at new strains with superior antifungal activity.
EcoSoils Systems, San Diego, California, has a unique approach to address the short-shelf-life problem of many microbials, such as Pseudomonas-based biofungicides. Industrial scientists ferment on-site continuously for control of turf diseases on golf courses. The company is applying the Bioject system to agricultural crops, especially for control of root rot diseases, such as Phytophthora root rot of avocado.
Natural products are another major source of new leads for pesticides. Most companies generate natural-product extracts either internally or from external sources and test them against test organisms or in mechanism-based screens. There are several advantages in using natural products directly as pesticides or as leads for new pesticides:
Large diversity of chemical structures.
High potential for finding new mechanisms of action.
Lower risk to environment.
Lower risk of toxicity to nontarget organisms.
Many of the new chemistries in fungicides, insecticides, and herbicides can trace their origins to natural products. For example, glufosinate is a derivative of bialophos, a natural product derived from Streptomyces viridachromogens (Leason et al. 1982). The methoxy-acrylate fungicides arose from strobilurin A, a natural product extracted from a mushroom (Clough et al. 1992). The active ingredient in the new spinosad insecticides used in cotton is isolated from the naturally occurring soil organism Saccharopolyspora spinosa (Thompson et al. 1994, Adan et al 1996, EPA