PCC Nov/Dec 2019

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