Agriculture Reference
In-Depth Information
(c) Is seedborne inoculum the principal source of infection or are other inoculum
sources more important?
For some plant diseases, seedborne inoculum is the only, or principal, source of
infection: if healthy seed is sown, disease does not develop in the growing crop. For
example, barley leaf stripe occurs only when seeds infected with the fungus
Pyrenophora graminea
are sown. There is no further spread of infection between
plants during the growing season and the disease can, therefore, be controlled very
effectively by sowing healthy seed, or by applying an effective seed treatment
fungicide to infected seed.
In contrast, a closely related fungus,
Pyrenophora teres
, can also be seedborne
and infected barley seeds can give rise to seedlings that show symptoms of net
blotch, but sowing disease-free seeds will not necessarily result in a crop free from
net blotch, since inoculum from other sources can establish the disease at any stage
of growth. In practice,
P
.
teres
present on old stubble, on volunteer barley plants and
in neighbouring crops is a much more important source of net blotch infection than
seedborne inoculum. Unlike leaf stripe, net blotch can spread between plants during
crop growth and, beyond the early seedling stage, the amount of net blotch in a
barley crop is rarely related to the initial level of inoculum in the seed sown.
Controlling seed infection, therefore, has little or no effect on net blotch at later
stages of crop growth. Control of seedborne inoculum is likely to be effective only
in situations where inoculum from other sources is relatively insignificant.
(d) Is the disease monocyclic or polycyclic?
Some seedborne pathogens, for example,
Ustilago nuda
,
which causes loose smut
disease of barley, have relatively uncomplicated life cycles and there is no spread
between plants during crop growth. The relationship between seed infection and plant
infection is relatively constant and it is only at flowering, when seed reinfection takes
place, that environmental conditions influence disease development. With loose smut,
cool moist conditions at flowering can lead to higher rates of reinfection because the
flowers remain open for a longer period (Hewett, 1978). With monocyclic diseases,
reducing or eliminating seedborne inoculum (for example, through appropriate seed
treatment) usually gives very effective control of the disease.
Polycyclic diseases, on the other hand, are those that, once established within a
crop, will multiply and spread during suitable conditions. They frequently cause
local lesions on infected plants and, as the disease develops, these lesions increase in
size and produce increasing quantities of inoculum, which is spread to other parts of
the infected plant, to neighbouring plants and sometimes to plants in neighbouring
crops. The relationship between seedborne inoculum and plant infection is complex.
Environmental conditions play an important role and inoculum from other sources,
including volunteer plants and old crop debris, complicate the seed-to-plant disease
relationship. Many economically important seedborne pathogens have a polycyclic
disease pattern. Limiting the effect of polycyclic diseases on crop performance by
controlling seedborne infection will only be effective if seedborne disease levels are
kept very low and inoculum is not introduced from other sources.
