Group Title: Circular
Title: Foliar and head diseases of sorghum in Florida
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Permanent Link: http://ufdc.ufl.edu/UF00014575/00001
 Material Information
Title: Foliar and head diseases of sorghum in Florida
Series Title: Circular
Physical Description: 6 p. (1 folded sheet) : col. ill. ; 28 cm.
Language: English
Creator: Kucharek, Tom, 1939-
Publisher: Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida
Place of Publication: Gainesville
Publication Date: 1992
 Subjects
Subject: Sorghum -- Diseases and pests -- Florida   ( lcsh )
Genre: government publication (state, provincial, terriorial, dependent)   ( marcgt )
non-fiction   ( marcgt )
 Notes
Statement of Responsibility: by Tom Kucharek.
General Note: Title from caption.
 Record Information
Bibliographic ID: UF00014575
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: ltqf - AAA7042
ltuf - AJQ8308
oclc - 29554329
alephbibnum - 001834182
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Foliar and Head Diseases of Sorghum in Florida1


by Tom Kucharek2


Sorghum is not a major crop in Florida but its
potential for growth is good if problems related to
markets, perceived nutritional value for livestock, and
diseases can be solved. Solving plant disease problems
begins with accurate identification of the causal
agents. In Florida-produced sorghum, several diseases
caused by fungi and one bacterial disease occur. The
purpose of this publication is to provide information
on the field diagnosis and control of diseases that
occur in leaves and seed heads of sorghum in Florida.
Root and stem diseases (e.g. Fusarium and Pythium
root rots) also occur but information relating to these
soil-borne diseases is limited at this time.

ANTHRACNOSE

Anthracnose is caused by the fungus Colletotrichum
graminicola. Anthracnose is the most common disease
of sorghum in Florida and has caused yield decreases
in excess of 50%. This fungus can cause seedling
blight, foliar leaf spots, root rot and stalk rot. Foliar
leaf spots represent the most predominate phase in
Florida. In drier production areas, the stalk rot phase
(red rot) may be more important than the foliar
phase.

Leaf symptoms of anthracnose appear as elliptical
small reddish-purple to tan (depending on the variety)
spots that expand to lengths of 1/8 to 7/8" (3 to 22
mm ). Commonly, lesions appear first in the midvein
(Fig. 1) and as the epidemic proceeds, abundant
lesions occur in the leaf blades and sheaths (Fig. 2).
Full-sized spots are usually tan in the center with a
distinct reddish-purple to brown border. Within the


tan center, fruiting structures (acervuli) of the fungus
are visible through a hand lens. The fruiting
structures contain a gelatinous mass of white spores.
Interspersed among the spores are dark spines
setaee). With an increase in lesions in leaf blades,
more lesions will be found in leaf sheaths, peduncles
(stem between uppermost leaf and the head), and
flower parts (e.g. glumes).


Figure 1. A lesion of anthracnose in the midvein of
a leaf

Lodging can be more severe if the foliar phase of
anthracnose is severe. Anthracnose causes lodging
indirectly because the leaf phase reduces the amount
of nutrients translocated to the stem or directly by the
infection of the stalk, causing a disease called red rot.


1. This document was published December 1992 as Circular 1073, Florida Cooperative Extension Service, Institute of Food and Agricultural
Sciences, University of Florida. For more information, contact your county Cooperative Extension Service office.
2. Dr. Thomas Kucharek, Professor and Extension Plant Pathologist, Plant Pathology Department, Institute of Food and Agricultural Sciences,
University of Florida, Gainesville.

The Institute of Food and Agricultural Sciences is an Equal Opportunity/Affirmative Action Employer authorized to provide research, educational
information and other services only to individuals and institutions that function without regard to race, color, sex, or national origin.
Florida Cooperative Extension Service / Institute of Food and Agricultural Sciences / University of Florida / John T. Woeste, Dean


UNIVERSITY OF FLORIDA LIBRARIES


~ 3Le







FOLIAR AND HEAD DISEASES OF SORGHUM IN FLORIDA


In addition to the anthracnose fungus, other fungi are
likely to be associated with stalk rotting and lodging.


forage sorghum types tend to be more resistant to
anthracnose than grain sorghum types.

Frequent rains or irrigation during the summer
months in north Florida, when night temperatures are
warm, provide optimum conditions for infection,
symptom development and disease spread. Leaves of
susceptible varieties with numerous lesions turn
brown prematurely and significant yield losses result
(Fig. 3).


lesions of anthracnose in leaves


Important sources of inoculum (spores) to initiate the
leaf phase of anthracnose include susceptible crops or
weeds or crop refuse from those susceptible plants.
The anthracnose fungus also causes disease in grass
species such as Johnsongrass, Sudangrass, sorghum-
Sudan hybrids, broomcorn, corn, rye, sugarcane, and
other grasses. Thus, inoculum for a sorghum crop
may originate from these other plant species or their
residues associated with the soil, particularly if the
crop refuse is near the soil surface. However, in some
studies, anthracnose was less severe in no-till
plantings compared to conventionally tilled plantings.

Seed transmission of the anthracnose fungus has been
demonstrated. However, this source of inoculum is
not likely to be as important as that from the field
unless the inoculum associated with the seed is a new
or different pathogenic race for sorghum or the
planting site is initially void of inoculum.

Isolates of the anthracnose fungus from one grass
species may or may not infect another grass species.
Within those isolates that infect sorghum, pathogenic
races of this fungus exist. Thus, varieties that are
resistant to the isolates in one location may be more
susceptible in other isolates in another situation. For
example, in Florida, some varieties have been highly
resistant to anthracnose (Fig. 3.) in some tests but in
tests conducted in later years, those same varieties
were more susceptible. Presumably shifts in
pathogenic races occurred. Interestingly, in Florida,
/) I


Figure 3. Resistant (left) and susceptible (right)
varieties to anthracnose


Control of anthracnose includes: 1) crop rotation 2)
use of resistant varieties; 3) suppression of grass
weeds and volunteer sorghum in and around the field;
4) burial of old sorghum debris by plowing (the need
for this is conjecturable); 5) plantings in mid April to
early May usually have less damage from anthracnose
than later plantings in Florida, and 6) avoidance of
field operations when leaves are wet.

ZONATE LEAF SPOT

Zonate leaf spot is caused by the fungus
Gloeocercospora sorghi. It has not been a major
problem in Florida but in variety evaluation tests,
some varieties were clearly more susceptible to zonate
leaf spot than other varieties. Young lesions or small
lesions may appear similar to those caused by the
anthracnose fungus. However, large lesions are
distinctive with their circular alternating bands of
white or tan with bands of reds, purples, or browns
(Fig. 4).


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FOLIAR AND HEAD DISEASES OF SORGHUM IN FLORIDA


Figure 4. Zonate leaf spot


Frequent rains or irrigation coupled with warm nights,
such as those during north Florida summers, are
conducive for development and spread of target spot.
Equipment moving through the field could also serve
as a mechanism for spread, particularly when the
leaves are wet. Spores are produced in a gelatinous
matrix on the surface of the leaf lesions and on
overwintering sclerotia (an aggregation of fungal
hyphae into a cushion-like structure with an outer
rind). When leaves are wet, spores germinate by
forming filament-like structures which penetrate the
leaf surface. Tiny dark, pinpoint lesions may be seen
in the leaf 24 hours after the infection begins.

Other host plants for the zonate leaf spot fungus
include corn, Sudangrass, sorghum-Sudangrass
hybrids, Johnsongrass, broomcorn, napiergrass,
sugarcane, bentgrass, bermudagrass, and possibly
other grasses. The fungus survives in old crop refuse
or in the soil as black sclerotia (masses of fungal
hyphae bunched together) formed in infected leaf
tissue. Crop rotation, use of resistant varieties, burial
of old crop debris by plowing, and avoiding field tasks
when the leaves are wet should reduce this disease.

ROUGH SPOT

Rough spot is caused by the fungi Ascochyta sorghina
and A. sorghi. The former species is thought to
predominate in Florida. Most varieties are not highly
susceptible but occasionally susceptible varieties
appear in Florida. Rough spot could be as damaging
as anthracnose in Florida if susceptible varieties were
grown. Old crop residues and seed are reported to be


sources of inocula. Spores of this fungus are produced
within flask-shaped structures (pycnidia) and are
spread primarily by rain or irrigation splash. However,
physical contact with field equipment could serve as
another method of spread, particularly when the
leaves are wet. Little is known about the life cycle of
this disease. Rough spot has occurred in
Johnsongrass, Sudangrass, and wild sorghum types.

Symptoms of rough spot occur in leaf blades and
sheaths, peduncles, stalks and certain flower parts
(e.g. glumes). Leaf symptoms from other geographical
areas have been described as beginning as small
circular to oblong lesions that are tan-colored in the
center with a distinctive or well-defined border. Then,
the spots enlarge to oval to elliptical blotches that are
1/2 to 1" (1-2.5 cm) long and are gray, red-purple,
yellow-brown in color. In Florida, rough spot appears
similarly except the color of the lesions tended to be
dark brown to black and the tan center during early
lesion formation is not apparent (Fig. 5). A mild
irregular yellow area around the blotch may also be
present (Figs. 5 & 6). Also, in Florida, the margins
of lesions are somewhat feathered (Figs. 5 & 6). The
most distinctive feature of rough spot is the
sandpapery feel of the lesion because of the
numerous spore-bearing structures (pycnidia) on the
leaf surface (Fig. 6).


figure 5. Rough spot in leaves


Control of rough spot includes the use of 1) crop
rotation, 2) deep plowing of old crop residues, 3) use


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FOLIAR AND HEAD DISEASES OF SORGHUM IN FLORIDA


of resistant varieties, and 4) avoiding field operations
when leaves are wet.


-I
Figure 6. Rough spot with pycnidia in leaf


NORTHERN CORN LEAF BLIGHT
(LEAF BLIGHT)

Northern corn leaf blight (NCLB) is caused by the
fungus Exserohilum turcicum (Helminthosporium
turcicum). This fungus can cause a major foliar
disease in corn (field and sweet), but it has been seen
only on occasion in sorghum in Florida. Johnsongrass,
Sudangrass, teosinte, and gamagrass are also hosts for
this fungus. In other geographical areas, losses near
45% have occurred from NCLB in sorghum.
Pathogenic races occur but they have not been
classified for isolates that infect sorghum.
Aggressiveness of those races that infect corn or other
grasses has not been determined in sorghum.

This fungus produces spores conidiaa) on leaf lesions
and crop refuse. When wind disseminates these
spores to susceptible leaves, infection of the leaves
occurs when a filament from the germinating spore
penetrates the leaf surface. Leaf moisture is required
for spore germination and infection. Within three to
six days after infection, lesions appear and between six
and 14 days, a new crop of spores are formed on the
lesions. Spore production and lesion development
occur between 50 to 94 F (10 to 34 C) but optimum
temperatures for disease development are near or
slightly below 61 F (16 C) for a minimum daily


temperature when the average daily temperature is
near 72 F (22 C). This fungus can also produce thick
walled spores called chlamydospores which lengthens
the survival time of the organism in soil.

Mature lesions vary in size but may be from 1 to 6"
(2.5 to 15 cm) long and up to 1/2" (1.3 cm) wide (Fig.
7). Lesions tend to be cigar-shaped, tan to brown in
the center. Sometimes lesions are multiple-pointed at
the tips. A lesion may or may not surrounded by a
dark brown-reddish purple border or a narrow band
of water soaking (Fig. 7).





















Figure 7. Northern corn leaf blight (leaf blight)


If control is needed, use resistant varieties, crop
rotation with non grass crops, and bury old sorghum
crop debris. Also, destroy Johnsongrass and
volunteers of susceptible crops in the vicinity.

BACTERIAL LEAF STRIPE

Bacterial leaf stripe (bacterial stripe) is caused by the
bacterium Pseudomonas andropogonis. It occurs in
Florida and elsewhere but is not a major problem.
Other hosts include corn, Johnsongrass, Sudangrass,
teosinte, sugarcane, clover, velvet bean, and other
Sorghum spp. Sorghum varieties differ as to
susceptibility but apparently most are resistant.

This bacterium can grow from 40 F to 100 F (5 to 38
C), but the optimum for growth is near 72 to 86 F (22


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FOLIAR AND HEAD DISEASES OF SORGHUM IN FLORIDA


to 30 C). Rain, irrigation water, or contact are the
main methods for its spread. The bacteria enter the
leaves through breathing pores stomatess) and can
live in old crop debris. Infection of the kernel,
peduncle, rachis, and inner stalk tissue can occur.
Apparently, this bacterium can be seed borne.

Lesions often first appear as reddish-purple to tan-
brown somewhat linear spots less than 1/2 (1.3 cm)
long. Lesions tend to be interveinal and may attain
lengths of 8" (20 cm) with the lesion color being fairly
uniform throughout (Fig. 8).

Control methods include: 1) crop rotation, 2) resistant
varieties, 3) plowing down old crop debris, and not
working in the field when it is wet.


SORGHUM RUST

Sorghum rust, caused by the fungus Puccinia purpurea,
has not been a major problem in Florida. Sorghum
rust produces slightly elongated raised pustules in
leaves and in the outer tissue of the peduncle (head
stem). The pustules in leaves are typically not more
than 2 mm long in leaves, and often surrounded by a
reddish-brown to tan halo. Pustules in the peduncle
may be longer and linear in appearance or within oval
red to brown lesions.

Within the raised pustule, a red to orange brown
mass of spores (urediospores) will be exposed if the
peridium (covering) of the pustule has ruptured. If
the peridium has not ruptured, it can be cut with the
tip of a pocket knife to expose the urediospore
masses. These spores are dispersed primarily by wind.
Spores will germinate on wet leaves by forming a
small filament that penetrates the leaf. Approximately
10 to 14 days after infection, new pustules with spores
are formed. Later, teliospores, a different spore type,
may form within these pustules or new pustules. In
mass, the teliospores are dark brown to black.

This rust fungus also infects Oxalis comiculata, a type
of sorrel, where a third type of spore aeciosporee) is
produced. The importance of this latter spore stage in
the field for sorghum rust is not known.

Little is known about this rust fungus, but some
indications suggest that it is more likely to cause
disease during cool, wet conditions. Some have
suggested that forage sorghums tend to be more
susceptible than grain sorghum types. If this disease
becomes a problem, resistant varieties would be the
primary control.

SORGHUM DOWNY MILDEW

Sorghum downy mildew is caused by the fungus
Peronosclerospora sorghi. In Florida, it has occurred in
sweet corn and an interspecifically derived sorghum-
related weed (Sorghum almum) in central Florida. It
has not occurred in commercial grain or forage
sorghum up to this time.


Page 5








FOLIAR AND HEAD DISEASES OF SORGHUM IN FLORIDA


HEAD MOLD

Head mold is caused by many fungi. Fusarium spp.
(e.g. F. moniliforme, Fig. 9), Curvularia spp.,
Colletotrichum spp.,Altemaria spp., Helminthosporium
spp. and other fungi have been associated with head
mold in Florida. These fungi can survive in old crop
debris and in association with other crops. This
complex disease has been severe in some plantings,
particularly in varieties with tight panicles (heads) or
some non-bird resistant types. Varieties with white to
yellow to bronze seed coats tend to be more
susceptible to head mold than those with red seed
coats. Florida's warm and wet summer weather is
conducive to head mold. Infection begins during
flowering and continues throughout grain filling.
Feed value for livestock is reduced significantly and
some of these fungi (e.g. Fusarium spp.) may produce
toxins that poison livestock.

Head mold is evident by the abnormal colorations and
moldy growths on the head. Fusarium spp. typically
cause a white to pink coloration on the outside of the
flower and seed parts (Fig. 9). Also, inner tissues of
the flower branches and head stalk may be discolored
with shades of red to dark brown.


Controls include: 1) planting so that flowering is
initiated during drier months, 2) reducing insects such
as the sorghum midge, and 3) using resistant varieties.


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Page 6




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