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Field Crops
Cotton
Symptoms, source of inoculum and management of cotton diseases.
Disease
Seedling Diseases
(Rhizoctonia spp.,
Pythium spp., Fusarium
spp., and other fungi)
Symptoms: Loss before emergence is characterized by a rot of the seed or seedling. After
emergence, affected seedlings have dark lesions on the stem, often girdling the stem and
extending downward into the root system. Older plants have reddish-brown, sunken
lesions near the soil line.
Source of Inoculum: Some of the organisms causing seed rot and seedling diseases may be
carried on the seed coat while others persist in the soil.
Management: Use only high-quality seed. Plant seed only when soil temperatures at a 4inch depth reach 68 F for 3 to 4 days. Plant fungicide-treated seed to a weed-free seedbed.
Plant at proper depth for soil type and weather conditions. See table on fungicides.
Leaf Spots (Alternaria
Symptoms: Brown to red to tan circular lesions on leaves that may occur at any stage.
spp., Cercospora
Lesions may be up to ¾ inch, and margins usually appear reddish to purple. As the diseases
gossypina, Stemphylium progress, centers of lesions may appear sooty due to sporulation of the pathogen and
spp., and other fungi)
eventually fall away resulting in a “shothole” appearance of foliage. In cases of heavy
infestation, lesions may overlap and cause premature defoliation resulting in decreased lint
yields.
Source of Inoculum: Fungi overwinter in previous crop/weed debris.
Management: Plow under infected plant debris. Plant high-quality, acid-delinted,
fungicide-treated seed. Avoid nutrient stresses (particularly Potassium), drought stress,
other pest damage and herbicide injury. On highly susceptible varieties, foliar fungicides
may reduce incidence and severity, but economic benefit is rare.
Target Spot (Corynespora Symptoms: Brown to tan circular lesions on leaves usually beginning at bloom or canopy
cassiicola)
closure. Lesions usually will not have reddish to purple margins when found low in the
canopy and will have a “bullseye” appearance. Target spot lesions are usually larger than
other leaf spot lesions. The disease progresses from low in the canopy upward, and severe
defoliation may occur.
Source of Inoculum: The pathogen will overwinter in crop debris. Soybean is an alternative
host and may harbor the pathogen.
Management: Destroy debris. Some varieties may tolerate target spot better than others.
Avoid rank plant growth. Scout closely prior to and after canopy closure. Scout nearby
soybeans for target spot. Foliar fungicides may lower disease incidence and in some cases
preserve yield. Fungicide coverage is key, and applications should be made by ground with
a minimum total volume of 10 gal/A with high pressure, hollow cone or flat fan nozzles.
Verticillium Wilt
Symptoms: Leaf margins and between veins have pale yellow markings. Severely affected
(Verticillium sp.)
plants shed the young bolls. Light brown discoloration occurs in the interior woody tissue
of the stem with little to no discoloration just beneath the bark. Disease occurs in cool
weather with or without nematodes.
Source of Inoculum: Fungus lives indefinitely in the soil.
Control: Rotate with soybeans, sorghum or small grains.
Fusarium Wilt
Symptoms: Plants usually are stunted and may fruit early. Leaves may turn yellow, wilt and
(Fusarium sp.)
drop. Brown to dark-brown discoloration occurs on woody tissue just beneath the bark. It is
more severe on sandy soils, during hot weather and when root-knot or reniform nematodes
are present. (See root-knot nematodes below.)
Source of Inoculum: Fungus lives indefinitely in soil. Nematodes, likewise, live from year to
year in the soil.
Management: Use of tolerant varieties. Most recommended varieties exhibit tolerance to
Fusarium wilt. Under more severe conditions, use recommended nematicides. (Refer to
table on nematode control in field crops.)
Field Crops
Cotton
Symptoms, source of inoculum and management of cotton diseases.
Disease
Root Knot Nematodes
(Meloidogyne sp.)
Symptoms: Root systems are knotted or galled. Plants are stunted, slow growing and low
yielding. Usually associated with a high incidence of Fusarium wilt. Damage from RKN is
most severe on sandy soils. (See Fusarium wilt above.)
Source of Inoculum: Root-knot nematodes live from year to year in the soil as eggs,
juveniles or adults.
Management: Use resistant variety. Apply nematicide. Refer to table on nematode control
in field crops. Rotate to resistant variety of an alternate crop.
Reniform Nematodes
Symptoms: These nematodes cause severe stunting, reduced boll set and tight, locked bolls.
(Rotylenchulus sp.)
Root systems are restricted but not knotted. May be found in mixed to heavy soil.
Source of Inoculum: Reniform nematodes live from year to year in the soil.
Control: Apply nematicides. Refer to table on nematode control in field crops. There are no
resistant varieties. Rotate to non-host alternate crop.
Boll Rots
Symptoms: There are a wide range of symptoms since there are many organisms involved
(many fungi and bacteria) and many stages of boll development when damage may occur. Discolored, sunken areas
may develop on the boll surface. Seed and fiber may be damaged without surface lesions on
the boll. Fiber may be stained.
Source of Inoculum: Organisms causing boll rots may be carried over in the soil, on crop
debris or on the seed coat.
Management: Avoid rank growth, and control insects during boll development. Plantgrowth regulators may be helpful in reducing rank growth and in turn, boll rot incidence.
Table 2. Fungicides for the management of seedling and foliar cotton diseases.
Method of
Fungicide1 and Product Mode Rate3 (fl oz/A unless
Target Pathogen(s)
Application of Action Group2
otherwise specified)
In-furrow
Headline 2.08
11
4.3-11.4
Rhizoctonia solani, Pythium spp.,
spray
Priaxor
7+11
.1-.6 fl oz/1000 row ft Fusarium spp.
Quadris 2.08
11
.4-.8 fl oz/1000 row ft
& Generics
Reason 500 SC
11
0.45 fl oz/1000 row ft
Ridomil Gold
11
1-2
Rovral 4F
4
3.4-6.9
Terramaster 4EC
14
4-8
Uniform
4+11
4.4-6.5
In-furrow
Blocker 10G
14
10-20 lb/A
Rhizoctonia solani, Pythium spp.
granules
Ridomil Gold PC GR
4+14
7-10 lb/A
Ridomil Gold GR
4
1.3-2.5 lb/A
Foliar spray Headline 2.08
11
6-12
Alternaria spp., Cercospora spp.,
Priaxor
7+11
4-8
Stemphylium spp., Ascochyta
Quadris 2.08
11
6-9
spp. and Corynespora cassicola
Topguard
3
7-14
Anthracnose, Boll Rots, Rust
Twinline
3+11
8.5
1
Reference to commercial or trade names is made with the understanding that no discrimination is
intended nor endorsement of a particular product by LSU or the LSU AgCenter is implied.
2
Mode of action groups are determined by the Fungicide Resistance Action Committee (FRAC).
3
Rates are the amount of formulation (product) per acre unless otherwise indicated. Consult product label
for rates specific to row spacing, disease pressure or additional target pathogens.
Field Crops
Cotton
Table 3. Seed treatment fungicides used to manage cotton seedling diseases.
Fungicide1 and Product
Target Pathogen(s)3
2
Mode of Action Group
azoxystrobin
11
Pythium spp.
biologicals
NA
Rhizoctonia solani
difenconazole
3
Thielaviopsis basicola
etridiazole
14
Fusarium spp.
fludioxinil
12
fluopyram
7
A multitude of seed treatment options are available to cotton producers
ipconazole
3
using the fungicides listed in this table and biological compounds. Seed
iprodione
2
treatment fungicides are currently the preferred method of managing
mancozeb
M3
seedling diseases and nematodes because of convenience and efficacy.
mefenoxam
4
Seed treatments with multiple modes of action are recommended to
metalaxyl
4
manage a broad spectrum of seedling pathogens.
myclobutanil
3
PCNB
4
pyraclostrobin
11
thiram
M3
triadmenol
3
trifloxystrobin
11
1
Reference to commercial or trade names is made with the understanding that no discrimination is
intended nor endorsement of a particular product by LSU or the LSU AgCenter is implied.
2
Mode of action groups are determined by the Fungicide Resistance Action Committee (FRAC).
3
Consult product label for proper targeting of pathogens.
The cotton section was revised October 2015 by Dr. Trey Price.