Agriculture Reference
In-Depth Information
cortical tissue. Plants infected with white rot may have a cottony white or grey
mass of fungal mycelium on the stem base. This mycelium can infect adjacent
plants and, therefore, spread of the disease is facilitated by the high plant
densities typical of salad onion production. Black resting bodies about 0.5 mm
in diameter, termed sclerotia, are formed in the mycelium and on the base of
infected plants. The resting sclerotia of white rot can survive for more than 18
years in field soil.
The white rot fungus is highly specific to alliums, and the germination of
the sclerotia is stimulated by the characteristic volatile 'flavour' compounds
around their roots (see Chapter 8). The disease can occur on all the vegetable
alliums but tends to be more serious on garlic and onion than on leek. Field
infections can go unnoticed at first, but can reduce yields to uneconomic level in
4 years of successive onion crops. Once established, because of the persistence of
sclerotia in soil, infected land is usually abandoned for onion production.
Pink root
Infection by Pyrenochaeta terrestris results in characteristic pink-hued roots
which become darker red with time. Infected roots ultimately collapse and die.
The fungus is an important pathogen on a number of non-allium crops and
can survive in soil as various types of spore or on debris or roots of susceptible
crops. The pathogen produces several toxins involved in causing disease.
Fusarium basal rot
This disease is caused by Fusarium oxysporum f.sp. cepae , which is one of over
100 formae speciales (f.sp.) of F. oxysporum , many of which are pathogenic.
Cramer (2000) has reviewed the research on this disease. The primary source
of inoculum in the field is round, thick-walled chlamydospores. The pathogen
can cause disease at all stages of onion growth, from seedling to stored bulb
(see Fungal Diseases of Stored Bulbs, below). The fungus can penetrate the base
plate of alliums and cause a brown discoloration and rot, so that the shoot
easily detaches from the base. Infection can be increased by damage from pests,
and the disease sometimes occurs together with pink root.
Garlic and leek basal rot
The pathogen Fusarium culmorum has been recognized as the cause of a rot of
planted cloves, young plants and the baseplate and bulbs of garlic. The same
pathogen has been identified as the cause of a root and stem base rot of leeks,
which causes a reddish-purple discoloration at the base of the pseudostem. The
pathogen does not cause disease on onion or shallot. The disease was first
reported in garlic in California in 1976, and in leeks in Spain in 1996. Because
the leek disease was found in California on seedlings growing in soil-less
compost in glasshouses, it was thought likely to have originated from infected
seed, and therefore to be seed transmissible (Koike et al. , 2003).
Search WWH ::




Custom Search