CHEMICAL INTERACTIONS WITH BIOHERBICIDES TO IMPROVE EFFICACY

Bioherbicides can be defined as plant pathogens, phytotoxins derived f rom pathogens or other microorganisms, augmentatively applied to contr ol weeds. Although many pathogens with bioherbicidal potential have be en discovered, most lack sufficient aggressiveness to overcome weed de fenses to achieve adequate control. Plants use various physical and bi ochemical mechanisms to defend against pathogen infectivity, including callose deposition, hydroxyproline-rich glycoprotein accumulation, pa thogenesis-related proteins (PR-proteins), phytoalexin production, lig nin and phenolic formation, and free radical generation. Some herbicid es, plant growth regulators, specific enzyme inhibitors, and other che micals can alter these defenses. Various pathogens also produce chemic al suppressors of plant defenses. Secondary plant metabolism is a majo r biochemical pathway related to several defense processes. Increased activity of a key enzyme of this pathway, phenylalanine ammonia-lyase (PAL), is often a response to pathogen attack, as demonstrated in two weeds and their associated bioherbicidal pathogens: Alternaria cassiae on sicklepod and A. crassa on jimsonweed. Weakening of physical and b iochemical defenses, and lowering of resistance to pathogen attack, ma y result from reduced production of phenolics, lignin, and phytoalexin s caused by herbicides and other chemicals that affect cuticular compo nent biosynthesis and/or key aspects of secondary plant metabolism. Po tent PAL inhibitors [aminooxyacetic acid, alpha-aminooxy-beta-phenylpr opionic acid, and (1-amino-2-phenylethyl)phosphonic acid] have some re gulatory action on secondary plant metabolism and pathogenicity. Vario us herbicides and other chemicals dramatically affect extractable PAL activity levels and/or substantially alter PAL product production. Som e non-pathogenic organisms can alter herbicide efficacy, and some herb icides influence disease development in plants. Research has shown som e synergistic interactions of microbes and chemicals with relevance to weed control. Further research on pathogen interactions with agrochem icals (or other chemicals/regulators) could result in increased effica cy of pathogen-herbicide combinations, reduction of herbicide and path ogen levels required for weed control, and expanded pathogen host rang e.