Fungicides 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:
- Inorganic chemicals contain no carbon. Two chemicals are available
to amateur and professional growers. Commercially formulated
compounds of copper salts mixed with slaked lime (Bordeaux
mixture) form a protective barrier to fungi such as potato blight
when sprayed onto the leaf. Fine-grained (colloidal) sulphur controls
powdery mildews and apple scab.
- Organic chemicals contain carbon. Mancozeb (dithiocarbamate
group) and related synthetic compounds act protectively on a wide
range of quite different foliar diseases, such as downy mildews, celery
leaf spot and rusts, by preventing spore germination. Mancozeb is
available to both amateur and professional growers.
- Carbendazim (benzimidazole group) is available only to professional
growers. It is an example of a systemic ingredient, which moves
upwards through the plant’s xylem tissues, slowing hyphal growth and
spore production of fungal wilts, powdery mildews and many leaf spot
organisms. Damping off, potato blight and downy mildews are not controlled by chemicals within this chemical group. Many different
systemic groups are now used in horticulture.
- Myclobutanil belongs to the conazole group. It is available to both
amateur and professional growers. It is protectant and systemic, on
powdery mildews, black spot of rose and apple scab.
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.