Agriculture Reference
In-Depth Information
between AI and
B. byssoidea
(Yohalem
et al.
, 2003), and these authors have
suggested that the name
B. allii
be used for the hybrid. Occasionally, the leaf
pathogen
B. squamosa
causes neck rot on white skinned-bulbs. A sensitive
diagnostic method has been developed to distinguish between the three
pathogens that cause neck rot exclusively, and also
B. squamosa
and
B. cinerea
.
This method is based on characteristic restriction enzyme fragments of DNA
from a PCR-amplified section of DNA derived using primer sequences selective of
DNA regions that differ between these various
Botrytis
species (Nielsen
et al.
,
2002). The technique should facilitate studies on the epidemiology of the
pathogens and on the extent of seed contamination and infection by them.
The pathogens produce spores on dead leaf tissue, and these can invade the
senescent tissue that occurs at the tip of ageing onion leaves. The fungus then
spreads down the leaf into healthy tissue. It is latent in live green leaves,
causing no disease. By sequentially invading successive leaves when they start
to senesce, the pathogen ultimately enters those leaves which swell at the base
to form the outer fleshy scales of the bulb. It then remains symptomless in the
bulb for many weeks after harvest. Infection may also enter the bulb neck at
harvest time if flail-damaged neck tissue is exposed to spores, but such infection
is minimized by rapid drying of the necks after harvest (see Chapter 6). There is
no evidence of spread from bulb to bulb in store, and trials have shown a one-
to-one relationship between the number of bulbs rotting in store and the
number of infected plants present at harvest. The apparently progressive
increase in the number of bulbs that rot in store is simply the result of
differences in the time to start rotting between individual pre-infected bulbs.
The pathogens also invade inflorescences of onions and cause flower and
umbel blight, which can result in seeds superficially contaminated or deeply
infected. In a survey of seed production in the semi-arid inland regions of
Washington State, USA, where about 20% of the world's onion seed is produced,
the majority of onion seed-lots produced were contaminated with
B. aclada
and,
of these, most carried a low percentage of seeds with a deep-seated infection that
could not be destroyed by surface sterilization (du Toit
et al.
, 2004). Umbel and
flower infection occurred after the spathe had opened and had begun to senesce
and the florets had opened. The proportion of infected mother plants increased
during the growing period and nearly all were asymptomatically infected with
B. aclada
at flowering time, but there was no apparent relation between plant
infection and infection of harvested seeds. Infections were thought to enter seed
crops from the abundant airborne inoculum produced by nearby bulb crops,
volunteer plants, cull piles and debris.
The discovery that the disease was seed-borne proved crucial for its control in
the UK in the 1970s. By applying a coating of the systemic benzimidazole
fungicide, benomyl to seeds (1 g AI/kg seed), infection of the cotyledon from
inoculum on the seed-coat is prevented. Seed was the primary source of infection
that was resulting in annual losses of 15-50% of the stored bulb crop. Using this
well-targeted treatment, annual losses from neck rot were reduced to negligible
