In which of the following categories of plant diseases biological control is used generally: 1- soil borne 2- insect borne 3- water borne 4- air borne
Answers
Answer:
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Explanation:
CHAPTER 19
Plant Diseases
The risks of devastating plant disease epidemics have been considerably increased by
the following man-made changes in crop ecology (Zadoks and Schein, 1979): (a)
enlargement and aggregation of fields; (b) increase in the density of host plants; (c)
decrease in genetic diversity of host plants; (d) increase in monoculture; (e) increase
in fertilizer use; (f) increase in irrigated areas; (g) international exchange of contami-
nated material. Zadoks (1989) sums up the situation by stating that "whenever and
wherever a crop is grown for maximum yield, after correcting the deficiencies of soil
and climate by modern technology, the within-crop microclimate is suitable for the
major diseases ofthat crop".
Plant diseases are caused by (a) abiotic factors, which are non-infectious and com-
prise air, water and soil pollutants, nutrient deficiencies, etc., and (b) pathogens, that
are infectious. We are concerned here only with the infectious biotic agents.
Plant diseases and aridity
The conditions that are prevalent in dry regions have varying effects on plant dis-
eases. In semi-arid climates, summer heat and drought play a similar role to that of
winter cold in temperate climates in reducing drastically the amount of inoculum in
unirrigated crops, for which reason these crops are rarely attacked by highly
drought-susceptible bacteria (Palti and Shoham, 1979). However, many pathogens
survive the summer in a dormant state, or persist on crops grown under irrigation.
Categories
There are two main categories of pathogens from the point of view of their develop-
ment under arid conditions (Rotem and Palti, 1968):
(a) Pathogens well adapted to surviving dry spells or seasons. These are generally
weak or only moderately strong parasites, and include species of Alternaria and
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