| Climatic conditions in
the 2003 spring were very favourable for seedling disease in cotton.
In most areas, a large number of cold shock days in October were capped
off by a cold snap at the start of November.
These conditions were ideal for seedling disease and seedling mortality
as high as 60 per cent was observed in some fields. Symptoms consistent
with infection by Pythium were observed in many crops.
Seedling disease includes seed rots and pre and post-emergent damping-off
(collapse and death of seedlings). Seedling disease occurs when cotton
is attacked by a range of soilborne fungi, including Pythium, Rhizoctonia
and others. These other fungi may include the strains of Fusarium that
cause Fusarium wilt, or strains of Fusarium that do not.
Cotton is most susceptible to seedling disease pathogens in the early
stages of growth. Rhizoctonia and Pythium can infect seeds directly
when the soil is very cool and/or wet. If emergence is delayed by cool
conditions then the seedling is particularly vulnerable.
Newly emerged plants are vulnerable to post-emergent damping off because
the cells in the hypocotyl and tap root are still thickening their walls
and cementing themselves together. Later, the seedlings become resistant.
Damping-off by Rhizoctonia and Pythium rarely occurs in older seedlings
(after the two-leaf stage) except with very cool wet conditions. In
contrast, the Fusarium wilt fungus can kill cotton plants at any stage,
from sowing to harvest.
Symptoms
Post-emergent damping-off occurs when infections breach the hypocotyl,
effectively ‘strangling’ the plant. The black root rot fungus
(Thielaviopsis basicola) is frequently blamed for stand loss but, in
contrast to Pythium and Rhizoctonia, Thielaviopsis only infects the
outer layers of the roots and does not kill cotton seedlings.
Pythium and Rhizoctonia both cause sunken, orange/brown lesions on the
hypocotyl below soil level. If conditions are very wet these lesions
may progress above the soil surface. Pythium characteristically causes
a soft rot (see photo Page 12) but this will be less obvious in seedlings
that are dead and shrivelled (see photo this page). Rhizoctonia grows
as fine threads (hyphae) across the surface of the hypocotyl. When the
soil is dry enough, characteristic ‘dancing soil’ may be
visible on the hypocotyl (see photo next page).
Seed treatments
Each year, NSW Agriculture evaluates the effectiveness of seed-treatment
fungicides against seedling disease diseases. The standard fungicides
for cotton in Australia are PCNB (Quintozene) and metalaxyl (Apron,
Mantle).
Seed with these fungicides and others in various combinations was sown
in a field at the Australian Cotton Research Institute on September
29, 2003 and irrigated the next day. Seedling mortality was extremely
high if the seed was untreated (Figure 1).
Seedling mortality was much lower in any treatment that included metalaxyl,
which is active against Pythium but not Rhizoctonia. PCNB by itself
had no effect on mortality. So in this particular trial, the disease
pressure was dominated by Pythium. In other years and at other sites
Rhizoctonia has been the major contributor to seedling mortality and
sometimes Rhizoctonia and Pythium appear to contribute equally.
Although the standard combination of PCNB and metalaxyl performed well,
some of the other combinations performed as well or better (Figure 1).
PCNB was applied at 1.1 g/kg seed, whereas azoxystrobin and fludioxonyl
were each applied at 0.2 g/kg seed. So there is potential to reduce
the amount of fungicide entering the soil environment while maintaining
at least the same level of control as the current standard.
Control of seedling disease
Although the weather will always determine a ‘good’ or ‘bad’
year for seedling disease, several steps can be taken to minimise damping
off and ensure acceptable stands. These measures are outlined in the
Integrated Disease Management Guidelines produced by the Cotton CRC
and the Cotton R&D Corporation.
• Choose varieties with good seedling vigour;
• Use effective seed treatment fungicides;
• Sow into well-prepared, high, firm beds;
• Sow into moist soil in preference to watering up;
• Position fertiliser carefully to avoid ‘burning’
the roots (that is away from the planting line);
• Plant when temperatures are greater than 15°C and on
a rising plane;
• Incorporate rotation crop residues early (especially legumes);
and,
• Incorporate green manure crops at least four weeks before
sowing cotton and, if possible, with enough moisture to enable good
breakdown.
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