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Adverse environmental conditions can cause a range of symptoms, many of
which may resemble symptoms of diseases or pest damage. Before concluding
that a problem is due to a disease or disorder, it is important to consider
whether environmental factors may have caused the problem.
Adverse environmental conditions may include waterlogging of the soil,
extended dry conditions, cold temperatures in the field or in storage, frost
damage or lightning strike.
Symptoms on plants
Wilting may result from either too dry or too wet soil conditions.
If sustained waterlogging results in wilting, damage is likely to be severe, as
it is indicative of extensive root damage. In prolonged dry periods,
transitory wilting may be accompanied by yellowing and shedding of older leaves,
and general stunting. The symptoms may vary depending how rapidly
they develop, which in turn depends on the moisture-holding capacity of the
soil, the air temperature and humidity, and intensity of sunlight.
Dry soil may also increase development of nutrient deficiency symptoms,
depending on which nutrient is most limiting in the soil. This is because
the extent and activity of roots is limited in dry soil, and hence their ability
to supply nutrients for growth is reduced.
Sweetpotato is sensitive to chilling injury. Frost will kill
the foliage, and root damage will occur if soil temperatures drop below about 12oC.
Dry periods may increase the likelihood of chilling injury, as clear skies and
low humidity result in cooler-than-usual nights. Cold temperatures early
in the season can greatly reduce the development of storage roots. The
effect is similar to that of a waterlogging incident; both lead to poor storage
root initiation and poor yields, even though plants may appear to have
recovered.
Lightning strike can cause a patch of dead plants in the
field. Surrounding plants may have scorched leaves and necrotic lesions on
stems, leading to suspicion of a disease attack. However, the damage does
not continue to spread, and adjacent plants soon recover.
Herbicide injury can cause a range of symptoms, which may be confused
with nutritional or disease problems. They may include yellowing and death
of older or all leaves, bleaching or deformity of young leaves, scorching of
leaves on one side of the plant or the field, or general stunting. The
symptoms of specific herbicides are not described here. It is wise to ask
farmers about the use of herbicides in the area, if an unusual symptom is
reported.
Cracking of storage roots is common when a wet period follows
prolonged dry or cold weather. Cracks may appear partly healed over, or
they may become necrotic due to infection by fungi. The incidence of
cracking varies with genotype. Increased cracking can also be induced by
root-knot or
reniform nematodes, or by
boron
deficiency. It is likely that these disorders simply increase the
susceptibility of the roots to cracking in response to soil moisture change.
The presence of cracks alone does not allow diagnosis of which cause is
responsible.
Souring of storage roots results from waterlogging of the soil.
The roots become asphyxiated due to lack of oxygen. Ethanol accumulates in
the roots giving them a fermented smell. They tend to break down or
shrivel during storage. Shrivelled, alcohol-smelling roots may also
indicate
Rhizopus soft rot.
Excess soil water can also produce water blisters on the skin.
They are initially pale but darken as they dry out in storage. They may
appear similar to the blisters attributable to
boron
deficiency.
Chilling injury can occur if roots are stored at temperatures below
13oC. It can cause Hardcore, in which the flesh remains
hard after cooking. Chilling can also result in greater frequency of
decay, and poor sprout production. Roots should not be left in the field
overnight after digging if temperatures are low, as even a brief chilling can
reduce storage life and quality.
In storage, roots may become spongy, with pale mottled flesh which may
develop cavities. This internal breakdown results from excessive
loss of starch through root respiration, and/or excessive water loss. It
may be increased by insufficient curing, surface wounding or by chilling injury
at any time after harvest.
Clark, C.A. and Moyer, J.W. 1988. Compendium of sweet potato diseases. APS
Press. 74 p.
Contributed
by:
Jane O'Sullivan
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