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SOUTHERN BLIGHT IN PROCESSING TOMATOES:<br />
DIAGNOSIS,<br />
MANAGEMENT<br />
AND MONITORING<br />
By AMBER VINCHESI-VAHL | Ph.D. Area Vegetable Crops Advisor<br />
Colusa, Sutter and Yuba Counties, UCCE<br />
And CASSANDRA SWETT | Ph.D., Cooperative Extension Specialist,<br />
Vegetable and Field Crop Pathology Department of Plant Pathology<br />
Southern blight, caused by<br />
the fungus Sclerotium rolfsii,<br />
is a destructive crown rot<br />
disease that rapidly kills tomato<br />
plants. The fungus is favored<br />
by high temperatures (over 86°F),<br />
high soil moisture, dense canopies, and<br />
frequent irrigation. Southern blight<br />
survives in soil as hardened structures<br />
called sclerotia for at least five years.<br />
Each infected plant can produce tens of<br />
thousands of sclerotia and then become<br />
more widely distributed in a field with<br />
each successive field operation. Although<br />
this disease may initially only<br />
affect a few plants in the field, southern<br />
blight can be serious enough to cause<br />
significant yield loss within a season<br />
or two. With a host range of over 500<br />
plants, this fungus<br />
can easily persist<br />
from year to year in<br />
infected crop debris.<br />
Southern Blight<br />
Identification<br />
with other crown rotting diseases,<br />
like Fusarium crown rot. Severely<br />
affected plants can have vascular<br />
discoloration, which may be confused<br />
with Fusarium wilt. Accurate diagnosis<br />
is critical to effective control.<br />
You can distinguish southern blight<br />
in the field based on the following<br />
diagnostic traits, one or more of which<br />
may be present. Diagnosis requires<br />
looking at the soil around the crown of<br />
the plant, in addition to the plant itself.<br />
▶<br />
Small tan to reddish brown sclerotia<br />
form at the base of the plant and/or<br />
in the soil around the plant.<br />
▶ White fungal mycelium (thread-like<br />
strands) growing INTO the soil. No<br />
other fungus will grow extensively<br />
in the soil (Figure 1).<br />
▶ White fan like mycelium (threadlike)<br />
growing on the crown/affected<br />
tissues.<br />
▶<br />
▶<br />
Plants go from healthy to dead in<br />
less than a week—much faster than<br />
most crown rots (Figure 2).<br />
Circular disease patches. From a<br />
distance, they look like bands of<br />
dead plants.<br />
If none of these characteristics are<br />
present, the best way to diagnose the<br />
disease is to put infected tissue in a<br />
plastic bag on a moist paper towel<br />
Continued on Page 16<br />
Southern blight<br />
misdiagnosis is<br />
likely if it occurs<br />
in an area where it<br />
has not historically<br />
been an issue, like<br />
the Sacramento<br />
Valley. It can be<br />
easily confused<br />
Figure 1: Fungal mycelium growing into the soil.<br />
All photos courtesy of C. Swett.<br />
Figure 2: Rapid plant collapse and death caused by<br />
southern blight.<br />
14 Progressive Crop Consultant <strong>Nov</strong>ember / <strong>Dec</strong>ember <strong>2019</strong>