Agriculture Reference
In-Depth Information
Southern blight
The pathogen causing this disease attacks a wide range of crops in warm
climates. On onions its symptoms are dirty white spots on the outer bulb scales
and neck near the soil surface; these later become water-soaked and mushy. A
web of fungal mycelium may form on and around the bulb, and spherical,
brown sclerotia may form on the bulb. The fungal hyphae can penetrate onion
shoots directly. The pathogen can invade from plant to plant. The primary
inoculum can be mycelium in infested organic matter or from sclerotia.
Damping-off disease
In common with all crops, allium seeds and seedlings are susceptible to attack
after sowing and during germination, and to emergence by numerous species
of fungus in the families listed under damping-off in Table 5.6. These fungi are
ubiquitous in soil. Seeds are routinely film-coated with fungicides to reduce
damping-off (see below).
Pythium
spp. tend to be most destructive in cold, wet
soils, so good drainage helps to avoid infections. Details of the various fungal
species implicated and their life cycles are given in Schwartz and Mohan
(2008).
Smut and smudge
Onion smut caused by
Urocystis colchici
is a soil-borne disease, but because it
manifests primarily in the shoot it is discussed in the section on Fungal Leaf
Diseases. Onion smudge caused by
Colletotrichum circinans
invades bulb skins
from the soil, but is discussed in the section on Fungal Diseases of Stored Bulbs,
below.
Disease control
The techniques of disease control can be grouped under several headings
relevant to most of these soil-borne diseases.
Prevention of spread
All soil-borne diseases can be spread by anything that moves infected soil from
place to place. It is important to ensure that planting material is disease-free.
Transplants, sets and cloves should be raised on non-contaminated land and, if
necessary, inspected for freedom from disease. The movement of contaminated
soil from disease-infested land on boots, implements, etc. must be prevented.
Infected plant debris should be confined and the disease propagules destroyed
by high-temperature composting (Coventry
et al.
, 2002) or other means.
Cultural control
Crop rotation can be effective for the control of those root diseases with fairly
short-lived propagules. A 5- or 6-year gap between allium crops is effective for
