Agriculture Reference
In-Depth Information
percentages. For an outlay of a few thousand pounds for seed treatment, millions
of pounds worth of onions were saved from rotting. Trials showed that the
percentage of infected bulbs at harvest was directly proportional to the
percentage of infected seed seeds sown. However, a given percentage of infected
seed results in a higher percentage of infected bulbs in a wet growing season
than in a dry season (Maude, 2006). High humidity during the season of leaf
growth favours spread from leaf to leaf and, therefore, from plant to plant.
Sprays of spores of the competitive antagonist fungus,
Ulocladium atrum
prevented sporulation of
B. aclada
on dead onion leaves and remained effective
even if the leaves were dried periodically. The antagonist, therefore, shows
promise as a biocontrol agent for slowing secondary infection from leaf to leaf.
However,
U. atrum
did not prevent the asymptomatic spread of the pathogen
from necrotic leaf tips into adjacent healthy leaf tissue (Yohalem
et al.
, 2004).
The importance of seed as the primary source of inoculum remains
controversial, and probably depends on the prevalence of other sources of
inoculum in the locality (du Toit
et al.
, 2004). Spores may also emanate from
infected debris in soil or in dumps of onion waste: sclerotia can survive for more
than 2 years in such environments. Infected crops that overlap in time with
newly sown crops are another potential 'infection bridge' for the pathogen.
Such an overlap often occurs where spring- and autumn-sown onions are
grown together in the same locality. Infected onion sets which survive storage
could be a source of disease. The disease cycle is illustrated in Fig. 5.19.
The neck rot pathogens are rapidly destroyed by temperatures of 30°C or
above. Therefore, if onions are topped in the field, they should be quickly taken
into store and sealed at the neck by blowing warm air over the bulbs to destroy
any inoculum invading the neck. The pathogens cannot infect dry, senescent
leaf tissue, so that in dry climates, onions field-dried with the foliage intact are
not at risk. Treatments with fungicidal foliar sprays or fungicidal treatments to
harvested bulbs are not effective in disease control.
Black mould
In tropical regions where onions are frequently stored above 30°C, the fungus
Aspergillus niger
commonly blemishes stored bulbs. This fungus invades via the
neck, often via injured tissue when tops fall or are cut near maturity. Fleshy
scales can be infected if the dry, outer skin of the bulb is damaged. The infected
scales develop a white fungal mycelium on which sooty, black sporing bodies
(conidiophores) develop. The optimum temperature for the invasion and
growth of this pathogen is 32.5°C. Mould development on onion bulbs is
favoured by humidities of 80% or more in the store. Except on wounded bulbs,
the black, sporulating areas may be confined to the outer, fleshy scales
immediately below the dry skins. The fungus may occur in association with
bacteria that cause soft rot of bulbs (see Bacterial Diseases, above).
