Curative and Eradicative Effects of Fungicides

Abstract

Fungi and oomycetes are the causal agents of numerous fungal plant diseases, causing losses in agricultural production worldwide. Plant pathogenic fungi and oomycetes have coevolved with their plant hosts, and have developed extremely efficient mechanisms to cause an infection, to grow, multiply and spread on living plants. Most of these plant pathogens have an exceptional potential to reproduce within the plant host. For example, Pytophthora infestans can produce 50 000 to 117 000 sporangia per linear centimeter of infected potato stem length (Johnson et al., 2000). Larger apothecia of Sclerotinia trifoliorum can release about 4.7 million ascospores during their active period (Raynal, 1990). In late vegetation stages, the number of Fulvia fulva conidia per square centimeter of infected tomato leaf can exceed 17 000 (Veloukas et al., 2007), while Pseudoperonospora cubensis can produce even 195 000 sporangia per square centimeter of cucumber leaf seven days after infection (Wang et al., 2009). Considering the prevalence, ubiquitous nature and the ability of fungi and oomycetes to cause epidemics in relatively short period of time, disease management strategies are needed to secure the productivity in today's agriculture. Chemical control measures are particularly common in management of fungal plant diseases, and most of these measures rely on the use of fungicides. No matter of many differences in biology of different fungal and oomycetous plant pathogens, they share certain similarities. Most of the fungi produce various kinds of spores, which come in contact with plant tissue, germinate, and penetrate into the plant during the infection process. Some plant pathogenic fungi (e.g. Rhizoctonia solani, Sclerotium rolfsii, or Macrophomina phaseolina) rarely produce spores and infect plants through penetration of mycelia into plant tissue. After infection, fungi and oomycetes continue to grow as mycelium in or on plant organs, where they produce their new vegetative or generative, propagation or dormant structures. Nearly all fungicides used in agriculture today show their best effect if applied before the infection occurs. When present on the surface of plant organs, fungicides destroy fungal spores or suppress germination tubes, hyphae, and other fungal structures. As a rule, control of fungal diseases with fungicides is aimed to prevent an infection and subsequent disease development, and in such way the use of fungicides in plant protection differ from the use of antibiotics or antimycotics in medicine and animal health. However, in agricultural practice, there are many cases in which disease control measures are needed after the infection has occurred, after the pathogen is already sporulating or after