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growing season (e.g. Fusarium) while others—those causing powdery mildew—
usually complete several cycles within a growing season and subsequently cause
greater disease losses. The mode of spread of fungal pathogens is an important
determinant of disease epidemics. Soil-borne fungal pathogens, in general, move
slowly and therefore cause less and localized disease epidemics, while rusts and
powdery mildew are dispersed by the wind to several kilometers which can cause
severe disease epidemics in a shorter period.
Older plants are less susceptible to invasion by many fungal species due to thick-
ened and lignified cells which help to restrict the advance of fungal species from the
infection point (Al-Sadi et al. 2011b). The level of disease resistance in crops is
another determinant of disease epidemics. Cultivars with higher levels of resistance
can slow down disease progress and therefore epidemics. All these factors usually
interact together and become important determinants of disease levels as well as the
efficacy of management strategies implemented in a given area.
3 Plant-Pathogen Interactions
Fungal pathogens interact with their host plants in different ways and are usually
influenced by the type of pathogen, the host plant and environmental conditions. An
imbibing and germinating seed usually releases significant amounts of exudates into
the soil which has the potential to attract mobile pathogen propagules to the infec-
tion court and affect spore germination and germ tube growth (Martin 1995 ). The
type and quantity of these exudates vary with seed age and genotype as well as soil
moisture content (Stanghellini and Hancock 1971a). Exudates usually consist of
polysaccharides, amino acids, fatty acids and carbohydrates. Some may attract
propagules while others stimulate germination of fungal spores (Donaldson and
Deacon 1993 ). Zoospores of P. aphanidermatum are usually attracted by L-glutamine
in a chemotactic response, which has no effect on cyst germination while glucose
has the opposite effect. Also, uredospores and teliospores of Puccinia carthami are
stimulated by some specific volatile polyacetylenes compounds, released by host
plants, such as hydrocarbons, fatty acids, aldehydes and alcohols. Germination of
fungal spores is usually fast enough to occur within one hour of planting seeds, and
it can occur in response to temperature, nutrient availability and moisture in most
pathogens (see Martin and Loper 1999 ). The physical structure and chemical com-
position of the cuticle wax layer reportedly affects the germination of some fungi
such as Blumeria spp. (Zabka et al. 2008 ).
Most rust fungi penetrate the stomata while some penetrate the cuticle.
Penetration of Pythium species into roots is usually via breaks in the root surface or
by direct penetration. Penetration is attained within hours after germination allow-
ing fungal mycelia to grow through the epidermal and cortical cells. Infection of
seeds, seedlings or plants by fungal species typically results in seed decay, pre-
emergence damping off, post-emergence damping off, wilt, root rot, fruit rot, or
other diseases. Although some fungal species can apply mechanical force during
A.M. Al-Sadi