Page 8 - Vol.36-No.6 issue

CROP PROTECTION

id, powder-like seed dressing compris- ular and thin. Pathogens enter the young
ing the fungicide mixed with an inert and tender roots and stoots causing
powder, so that the formulation mixes seedlings to die soon after emergence
uniformly with and adheres to the seed. from the soil. Surviving plants will gener-
Extent to which this is achieved essen- ally lack vigour, be yellow or pale green
tially depends on surface roughness of in colour and fail to compete with healthy
the seed, the particle size of the seed seedlings. Severity and incidence of this
dressing formulation and the water solu- disease depends on three key factors:
bility of the fungicide ingredient. percentage of the grain carrying seed-
Best results are thus achieved with borne pathogens; soil temperature and
seeds possessing a rough seed coat, soil moisture content.
seed dressings in which the particles are Incidence of seedling blight is high-
of suitably small dimensions and a fungi- Rice paddies are inherently est in rice which is sown (seeded) early
cide which is sparingly soluble in water wet places. Fungicides used and especially if the soil is inappropri-
so that the active ingredient is released to dress rice seed should be ately cold and damp. However, disad-
by the action of moisture over a period sparingly soluble in water so vantages of early rice seeding can be
of time. The seed dressing should stay they are released slowly to partially compensated for by seeding
on the seed during the first crucial days protect rice seedlings and plants at a shallower depth. Conditions which
and weeks after sowing and during seed over a period of time tend to delay seedling emergence will
germination. (Picture Dr Terry Mabbett) generally aggravate seedling blight. In-
Many fungal plant pathogens are ei- cidence of the pathogens and seedling
ther seed-borne or exist in a quiescent Use of seed dressings minimises the blight disease can generally be reduced
state on plant debris in the soil. This is area of land exposed to pesticide. The by treating rice seed with an appropri-
especially so for annual crops. After har- fungicide is placed exactly where it is ate fungicide.
vest the fungus is forced to survive the required to achieve the most effective Seeds which are carrying these blight
ensuing break by surviving on or inside control at minimal cost. fungi often display spotted or disco-
the seed or on crop debris left behind in Seed dressing will provide enhanced loured hulls, although seed may be
the field. seed viability, vitality and seedling infected but still appears to be clean.
vigour, all of which are keys to successful Cochiobolus miyabeanus which is one
crop emergence and establishment. of the chief causes of seedling blight is
The seed dressing delivery system of- seed-borne. Seedlings attacked by this
fers the most even and uniform applica- fungus have dark areas on the basal
tion of chemical pesticide parts of the first leaf.
The soil-borne, seedling blight fun-
Provides more scope for a combina-
tion of treatments including: two or more gus Sclerotium rolfsii may kill or se-
fungicides, fungicide plus insecticide; verely damage a high incidence of rice
fungicide plus nutrient formulations seedlings after they emerge, especially
if conditions at emergence are warm
It may help in breaking seed dorman- and humid. Symptoms include cottony
Healthy rice seed together with cy to improve seedling emergence and white mould developing on the lower
a good fertiliser/nutrient profile establishment of the plant stand. parts of affected plants. This particular
for germinating and establishing blight may be controlled by immediate
rice seedlings will ensure a Seedling blight or damping flooding while treatment of the rice seed
vigorously-growing and uniform off disease in rice with an appropriate fungicide prior to
stand of rice planting will improve emergence and
(Picture courtesy Omex) Some of the biggest success stories establishment of the stand. Concentra-
in plant disease control using fungicide
Advantages of seed seed dressings have been achieved in
small grain cereals including wheat, bar-
dressings ley, oats and of course rice.

Seed treatment using fungicide dress- So why are fungicide seed dressings
ings has some advantages over other so applicable to protection of rice, espe-
pesticide application methods such as cially in the early stages of seed germi-
foliar application using liquid sprays. nation and seedling establishment? Rice
These include: is inherently susceptible to a collection
Seed is protected during storage and of soil-borne and seed-borne fungal
post-planting in the soil. and fungus-like pathogens which attack
the plant during and after the germina-
Fungicide seed dressing provides an tion stage to cause a disease commonly
immediate reduction in disease inocu- known as seedling blight or damping Healthy rice seed will germinate
lum at the very start of the crop cycle off. Implicated pathogens include vari- to produce vigorous fast-
Seed dressings are considerably more ous species of Cochiobolus, Curvularia, growing rice plants which are
targeted in application and avoid the Fusarium, Rhizoctonia, Sclerotium and better able to compete with
problem of spray drift and the environ- Pythium. weeds like wild grasses and the
mental consequences of pesticide spray Stands of rice affected by seedling sedges shown here
landing off target blight are characteristically spotty, irreg- (Picture Dr Terry Mabbett)
6 Vol. 36 No. 6