Laboratory studies to assess the risk of development of resistance to zoxamide.

Laboratory studies were conducted to evaluate the risk of developing field resistance to zoxamide, a new Oomycete fungicide which acts on microtubules. Zoxamide, metalaxyl and dimethomorph were compared with respect to the ease with which fungicide-resistant mutants could be isolated and their level of resistance. Attempts to generate mutants of Phytophthora capsici and P infestans with resistance to zoxamide by mycelial adaptation on fungicide-amended medium were unsuccessful. Similarly, changes in sensitivity to zoxamide were small (resistance factors < or = 2.2) in mutants of P capsici isolated by chemical mutagenesis of zoospore cysts. In parallel experiments with metalaxyl, highly resistant mutants were obtained using both adaptation (P capsici or P infestans) and chemical mutagenesis (P capsici). For dimethomorph, chemical mutagenesis (P capsici) yielded moderately resistant mutants (maximum resistance factor = 20.9), and adaptation (P capsici or P infestans) did not induce resistance. It is proposed that failure to isolate mutants resistant to zoxamide results from the diploid nature of Oomycete fungi and the likelihood that target-site mutations would produce a recessive phenotype. Our studies suggest that the risk of a highly resistant pathogen population developing rapidly in the field is much lower for zoxamide than for metalaxyl. However, as with any site-specific fungicide, appropriate precautions against resistance development should be taken.

The chemical and biological properties of methoxyfenozide, a new insecticidal ecdysteroid agonist.

Methoxyfenozide [N-tert-butyl-N'-(3-methoxy-o-toluoyl)-3,5-xylohydrazide; RH-2485] is the newest diacylhydrazine insecticide to reach the marketplace. It binds with very high affinity to the ecdysone receptor complex (EcR:USP) in lepidopteran insects [Kd = 0.5 nM (Plodia)], where it functions as a potent agonist, or mimic, of the insect molting hormone, 20-hydroxyecdysone (20E). Methoxyfenozide exhibits high insecticidal efficacy against a wide range of important caterpillar pests, including many members of the family Pyralidae, Pieridae, Tortricidae and Noctuidae. It is most effective when ingested by the target caterpillar, but it also has some topical and ovicidal properties. It is modestly root systemic, but not significantly leaf-systemic. Evidence collected to date indicates that methoxyfenozide has an excellent margin of safety to non-target organisms, including a wide range of non-target and beneficial insects.

Structure-activity relationships of phenylcyclohexene and biphenyl antitubulin compounds against plant and mammalian cells.

Phenylcyclohexenes (PCHs) [e.g., trans-4-nitro-5-(2,3,4-trimethoxyphenyl)cyclohexene, 2d] were found to bind weakly to the colchicine site of bovine tubulin, but are the first mimics of colchicine found to have high activity towards plant cells. Structure-activity relationships for PCHs and biphenyl AC-ring analogues of colchicine (e.g., 2,3,4,4'-tetramethoxy-2'-methyl-1,1'-biphenyl, 3e) are discussed.

Identification of ceramide-phosphorylethanolamine in oomycete plant pathogens: Pythium ultimum, Phytophthora infestans, and Phytophthora capsici.

Cellular lipids were extracted from three species of Oomycete plant pathogens (Pythium ultimum, Phytophthora infestans, and Ph. capsici) and analyzed via normal-phase high-performance liquid chromatography with flame-ionization detection. The most abundant polar lipids in each of the three species were the polar membrane lipids, phosphatidylethanolamine (PE), phosphatidylcholine, and a phosphosphingolipid that eluted soon after PE. Structural analysis via mass spectrometry and nuclear magnetic resonance spectrometry revealed that the phosphosphingolipid was ceramide phosphorylethanolamine (Cer-PE). The most abundant molecular species of Cer-PE in P. ultimum had a molecular weight of 670.5, contained an unusual 19-carbon branched triunsaturated sphingoid (C19-delta 4, 8, 10, 9-methyl long-chain base) and palmitic acid as the amide-linked fatty acid. The most abundant molecular species of Cer-PE in Ph. infestans had a molecular weight of 714.5, contained a common 16-carbon 1,3 di-OH sphingoid, and erucic (cis 13-docosenoic, C22-delta 13) acid as the amide-linked fatty acid. The Cer-PE in Ph. capsici comprised a mixture of each of the two molecular species found in P. ultimum and Ph. infestans.

RH 5849, a Nonsteroidal Ecdysone Agonist: Effects on Larval Lepidoptera.

The ecdysone agonist RH 5849 (1,2-dibenzoyl-1-tert-butylhydrazine) causes the premature initiation of molting at all stages of larval development of the tobacco hornworm, Manduca sexta. This phenomenon occurs without an increase in the endogenous ecdysone (20-hydroxyecdysone) titers. RH 5849 likewise provokes the initiation of molting in larval abdomens in the absence of a source of endogenous hormone. Although substantially less active than 20-hydroxyecdysone in vitro, RH 5849 was 30 to >670 times as active as the authentic molting hormone in bioassays with isolated larval abdomens or intact hornworms. This reversal in potency can be attributed to the superior transport properties and metabolic stability of RH 5849 relative to 20-hydroxyecdysone. Thus RH 5849 and its analogs are relatively persistent ecdysone agonists that halt feeding in larval lepidoptera by forcing an ultimately lethal, developmentally premature molt.