September/October 2017
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Continued from Page 13<br />
taproots (Table 2, Pg. 14). The<br />
two wilts may be differentiated<br />
in that Verticillium wilt symptoms<br />
usually show on mature<br />
plants that are close to harvest,<br />
while Fusarium wilt symptoms<br />
can occur on both young and<br />
mature plants. However, the<br />
buildup of ammonium in soils<br />
can damage the central cores<br />
of lettuce taproots. Lettuce<br />
roots exposed to high levels<br />
of ammonium therefore may<br />
look like a Fusarium wilt case.<br />
Again, to know precisely the<br />
cause of declining lettuce plants<br />
that have vascular discoloration<br />
in taproots, one must have the<br />
plants tested by a pathology lab.<br />
Symptoms Fusarium Verticillium Ammonium<br />
Yellowed leaves Yes Yes Yes<br />
Wilted leaves Yes Yes Yes<br />
Collapsed plants Yes Yes Yes<br />
Vascular discoloration Yes Yes Yes<br />
Decayed crowns No No No<br />
Rotted roots No No No<br />
Symptomatic young plants Yes No Yes<br />
Symptomatic mature plants Yes Yes No<br />
Table 2. Similarity of disease symptoms for lettuce affected by Fusarium wilt, Verticillium wilt, and<br />
ammonium toxicity.<br />
Strawberry growers face a<br />
similarly daunting task of trying<br />
to identify soilborne diseases.<br />
Three significant diseases have<br />
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virtually identical symptoms of off-color, gray<br />
green foliage, wilting of leaves, collapsing of<br />
plants, dark internal discoloration of strawberry<br />
crowns, and eventual death of the plant (Table<br />
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3, pg. 15). Because Macrophomina crown<br />
rot, Fusarium wilt, and Phytophthora<br />
crown rot share all of these symptom<br />
features, it is not possible to know which<br />
one is present in the field. Two of these<br />
problems, Macrophomina crown rot and<br />
Fusarium wilt (Photo 5), are very damaging<br />
to strawberry plantings and in addition<br />
have been steadily spreading; both pathogens<br />
are now found in all of the major<br />
coastal strawberry production regions in<br />
California.<br />
Why Accurate Diagnoses are<br />
Essential in Production<br />
Agriculture<br />
Some might feel that “a dead plant is<br />
a dead plant” and that knowing the exact<br />
name of the responsible pathogen or factor<br />
is not that important. As an extension<br />
plant pathologist, I would suggest that it is<br />
absolutely essential in today’s competitive<br />
agricultural world to know precisely the<br />
cause of a problem. Accurate pathogen<br />
identification always has been the first<br />
required step in fashioning an integrated<br />
pest management (IPM) program for dealing<br />
with plant diseases. Pathogen identification<br />
enables the grower to know which<br />
crops are susceptible to the agent, which<br />
crop cultivars might be advisable to plant<br />
due to genetic resistance, how the pathogen<br />
gets around and is spread, and other<br />
14 Progressive Crop Consultant <strong>September</strong>/<strong>October</strong> <strong>2017</strong>
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