USES AND CHALLENGES WITH NOVEL FUNGICIDES FOR PLANT DISEASE CONTROL
DI. VA. PRA - Patologia vegetale, Universita' di Torino, Via L. Da Vinci 44, 10095 Grugliasco (Torino), Italy
Fungicides remain vital for effective control of plant diseases. During the past few years, fungicide research has produced a diverse range of products with novel modes of action which are expected to have a significant impact on disease control in the next decade. New classes of fungicides include strobilurins, phenylpyrroles, anilinopyrimidines, phenoxyquinolines, oxazolidinediones, spiroxamine, diarylamines, scytalone dehydrase inhibitors, systemic acquired resistance (SAR) compounds. An important aspect has been the identification of suitable and novel target sites. The most recent developments in fungicide discovery originated from different approaches: some through traditional random screening, others from natural products, others through the exploitation of the SAR mechanism in plants.
Several new fungicides have been discovered through traditional random screening including famoxadone, spiroxamine, anilinopyrimidines, phenoxyquinolines, diarylamines. For example, regarding disease spectrum, famoxadone has a relatively broad spectrum, whereas quinoxyfen in the phenoxyquinolines is effective only on powdery mildews and carpropamid in the scytalone dehydratase inhibitors controls only rice blast.
Naturally occurring substances became important sources of antifungal agents: two of them pyrroinitrin, produced by Pseudomonas pyrocinia, and strobilurin A, produced by Strobilurus tenacellus, led to the development of two major classes of commercial products, phenylpyrroles and strobilurins, respectively. Phenylpyrroles include fenpicionil, used as seed dressing, and fludioxonil, applied as seed dressing and as foliar spray against Botrytis cinerea. Fenpicionil appears to affect glucose metabolism, although the target site has not been delineated. Azoxystrobin, kresoxim-methyl, and metominostrobin, the three announced strobilurins, taken together, have a wide spectrum of activity. Their fungicidal activity results from the inhibition of mitochondrial respiration. Their wide spectrum of activity, coupled with their novel mode of action, suggests a major impact on plant disease control.
New developments in the investigation of compounds with indirect mode of action led to the exploitation of functional analogues of salicylic acid. Among them, acibenzolar-S-methyl (CGA 245704 commercialized as Bion) has been developed: its unique feature is the ability to protect plants by fungal and bacterial infections. Initially, acibenzolar-S-methyl has been exploited on cereal powdery mildews, rice blast as well as other diseases such as tobacco blue mould. It fulfills all the criteria of a plant activator: it has no direct effect on pathogens and its mode of action is distinctly different from any other conventional fungicide. Due to its mode of action, it must be applied before the onset of disease. Plant activators are expected to provide new opportunities for effective and sustainable disease management in several crops. In recent years, the risk of resistance development has been investigated for new compounds. Baselines have been established for key pathogens in the case of most new fungicides. A better understanding of the population biology and genetics of important pathogens as well as new technologies derived from molecular biology will contribute to the development of techniques for early resistance detection. These will provide useful information for designing anti-resistance strategies to be applied from the start of commercial use for compounds considered to be at risk, thus limiting the problems encountered in resistance management in the past. In the meantime, compounds with indirect mode of action should, at least theoretically, remain excluded from resistance risk.
The increased availability of novel fungicides, characterized by new modes of action, will have a profound impact on disease control for certain crops, providing growers more latitude in their choices. Grapevine represents a very good example in this respect: the control of its three major fungal diseases will strongly benefit from the presence of many new classes of chemicals. Low mammalian toxicity and environmental impact are very important features of new fungicides. These characteristics will improve compatibility between fungicides and other control agents. Fungicides are expected to remain essential tools for plant disease management: their new features will present a challenge for optimal use under IPM strategies.