Fungicides
ContentFungicides must act against the disease, but not seriously interfere with plant activity. Protectant chemicals prevent the entry of hyphae into roots and the germination of spores into leaves and other aerial organs (see Figure 15.4). Systemic chemicals enter roots, stems and leaves, and are translocated to sites where they may affect hyphal growth and prevent spore production. Although there are many fungicidal chemical groups, four are chosen here as examples:
Resistance to pesticides The development of resistant individuals from the millions of susceptible weeds, pests and diseases occurs most rapidly when exposure to a particular chemical is continuous or when a pesticide acts against only one body process of the organism. Resistance, e.g. in powdery mildews, to one member, e.g. carbendazin, of a chemical group confers resistance to other chemicals in the same benzimidazole group. Growers should therefore follow the strategy of alternating between different groups and not simply changing active ingredients. Particular care should be taken with systemic chemicals that present to the organism inside the plant a relatively weak concentration against which the organism can develop resistance. Increase in dosage of the chemical will not, in general, provide a better control against resistant strains. Biological control, unlike chemical control, does not create resistant pests. |