4 Scouting for Diseases
Small grain diseases vary in intensity and severity
each year depending on the weather, cultural practices, variety of small
grain grown, and the presence of a disease-causing agent. Efficient disease
management of small grains requires that growers anticipate disease problems
well in advance. For example, planting disease-resistant varieties in the
fall will minimize disease problems the following spring and summer. Proper
crop rotations will also reduce the likelihood of disease occurrence in
the future.
Growers should have an organized method of scouting
fields and should take the time to learn how to identify disease problems
correctly. Small grain fields should be monitored during the season to
detect any developing problems. Learning about weather conditions that
favor diseases will help growers know when to scout fields or apply crop
protection chemicals. In order to justify disease control measures, identify
any diseases present and determine the severity of each. County Cooperative
Extension Service agents can provide further information on small grain
disease identification, scouting techniques, and a fist of resistant varieties
suited for your production area.
Beginning in mid-March, fields should be scouted
for diseases every week. Take care to identify the disease correctly so
that the proper fungicide can be selected (Order Extension Service publication
AG-419-7, Disease Identification, from Box 7603, NCSU, Raleigh,
NC 27695-7603).
Scouting Procedures for Common Diseases
A good disease-scouting procedure follows these steps:
1. Review the field history. Identify fields that have had chronic
disease problems. Make sure to check for these diseases when scouting.
2. Scout weekly starting in mid-March. Check the most disease-prone
locations first. Foliar diseases tend to be worse in thick growth. Row
ends where seeding and fertilizer rates are inadvertently high often have
the heaviest disease pressure. Soil-borne viruses often occur in low, wet
areas.
3. Identify all diseases present as soon as possible. Some diseases
increase quickly (i.e., rust) and must be dealt with promptly. Multiple
diseases sometimes require different control measures.
4. Map the areas where diseased plants are found to determine where
treatments should be applied, to monitor any disease increase, and to assist
in planning future crops.
Barley Yellow Dwarf (BYDV)
Barley yellow dwarf, also called oat red leaf, is
the most important virus disease of wheat, oats, barley, and rye in North
Carolina. The virus is transmitted by aphids from grasses such as orchard
grass, tall fescue, and ryegrass. It must survive in a living host and
cannot live in plant debris or the soil. Barley yellow dwarf virus is most
likely to occur after a warm fall and mild winter which favors grass (the
alternate host), cereal growth, and aphid multiplication. Symptoms are
often overlooked or mistaken for nutritional problems. Leaves may be shades
of yellow, red, or purple, especially from the tip to the base and from
the margin to the midrib (Figure 10
and figure 11). Plants are stunted
if infected early in the fall and are progressively less stunted if infections
take place as the plant matures.
Infected plants normally are found in small areas
(usually only a few feet in diameter) within the field (Figure
12).
Controlling aphids with insecticides has not proven
to be effective. The greenbug aphid is not a transmitter. The bird cherryoat
aphid (in the fall) and the English grain aphid (in the spring) are the
most important transmitters. There are no practical control measures at
this time.
Septoria Glume Blotch
Symptoms of septoria glume blotch may occur at any
time during the growth of the plant and on any portion of the plant (Figure
13 and figure 14). Lesions are
round to lens shaped and appear on the oldest leaves first. Lesions begin
with a water-soaked appearance, later drying and turning a yellow or red-brown
color. Tissue death eventually extends beyond the lesion, sometimes to
the entire leaf. Older lesions have small, dark, pimple-like spots called
pycnidia, which are diagnostic for glume blotch. The fungus is dispersed
by wind-blown rain. Wet, windy weather that favors spore dispersal increases
the severity of this disease. Dry periods not only prevent infection, but
halt disease development.
Loose Smut
Symptoms of loose smut occur between heading and
maturity. At first, the blackened, diseased heads are clearly visible among
newly emerged, green, healthy heads (Figure
15). Infected heads emerge slightly earlier than normal and have their
spikelets, except for a delicate membrane, entirely transformed into a
dry olive-black spore mass. The membrane tears easily as heads emerge,
and once the spores are dispersed by wind, all that remains is the stem
or rachis (stem of the head). Infections occur only during flowering and
are favored by wet weather and cool to moderate temperatures (61o
to 72o F.). Within one week after flowering, the ovary and attachments
become resistant. Infected seed appear normal. When infected seed are planted,
the fungus, which is found inside the embryo, will grow within the seedling
when it begins to germinate. Smutted grains appear only after seed heads
emerge from plants that came from infected seed. This means that an infection
is not seen until plants from the infected seed mature the following year.
Powdery Mildew
Lesions are first noticeable as white, powdery spots
on the lower leaves and stems (Figure
16). As the lesions mature they become darker, sometimes salmon colored,
with black spots (perithecia). If there is a heavy infestation, clouds
of white spores can be seen as someone walks through the wheat. Spores
are dispersed by wind. High humidity (with or without rain) and cool temperatures
(59o to 72o F.) favor disease development. The disease
slows markedly at temperatures above 77o F.
Root Rots and Seedling Diseases
Some areas of the field may be sparsely populated
with wheat. This may be noticed first as a weedy area. Seedlings
will be rotted or will have lesions and be generally unthrifty. Root rots
cause plants to be stunted, wilted and/or discolored. The roots will have
an unhealthy, darkened appearance. Since the fungi that cause these problems
are rather common in most fields, dispersal is not as important as with
other diseases. Exceedingly wet and cool weather favors root rot. Any conditions
that retard seed germination can result in seedling disease.
Rust
Lesions are small, circular, and vivid orange-red
in color (Figure 17). They may occur
on stems, but are most common on the upper surface of the leaves. When
heavily infected, the whole leaf will die. Winds can carry rust spores
for great distances. Rapid development occurs between 59o and
72o F. when moisture is available.
Scab or Head Blight
Scab is seen as prematurely bleached heads or spikelets
(Figure 18 and figure
19). If the rachis is infected, everything above that point will be
faded. Small dark spots (perithecia)
and superficial pink or orange fungal growth (mycelium and spores)
can be seen at the base of the spikelets. Only partly filled seed will
be found in the infected spikelets. The fungus is spread by air currents.
Warm (77o to 86o F.), moist weather favors scab.
Wheat Soil-Borne Mosaic Virus
The symptoms of Wheat Soil-Borne Mosaic Virus (WSBMV)
are most obvious in the early spring months. The virus which causes WSBMV
is transmitted by a soilinhabiting fungus present in many fields of small
grain. Symptoms often appear in low, wet areas, but may also cover all
or most of a field. Symptoms range from mild green to prominent yellow
leaf mosaic blotches, longer than wide (Figure
20). Plants may also be stunted. Symptoms usually go away once warm
conditions arrive in the spring. WSBMV also infects barley and rye,
Wheat Spindle Streak Mosaic Virus (WSSMV) looks
exactly like WSBMV and is transmitted in the same manner, but infects only
wheat. Control of these two diseases is difficult. Rotations are not effective
since the virus can remain infective in the soil for many years inside
the soil fungus. Oats do not show symptoms of WSBMV or WSSMV and may be
substituted for wheat.
Oat Soil-Borne Mosaic Virus, which infects only
oats, is transmitted by the same soil-borne fungus.
| Major Diseases and Management Methods | |
| Disease | Management Methods |
| Barley yellow dwarf | 1 .Plant as late as practical. |
| Septoria glume blotch | 1 .Apply seed treatment.
2. Use certified seed. 3. Plow under infested residue. 4. Lengthen rotation (plant crops other than small grains). 5. Keep potash, copper, and magnesium at recommended levels. 6. Use foliar fungicides. |
| Loose smut | 1 .Seed treatment (carboxin or triadimenol).
2. Use certified seed. |
| Powdery mildew | 1 .Plant resistant varieties.
2. Lengthen rotation (crops other than small grains). 3. Plow under volunteer plants where feasible. 4. Use foliar fungicides. 5. Apply seed treatment (triadimenol). |
| Root rots and seedling diseases | 1. Apply seed treatment.
2. Lengthen rotation (crops other than small grains). |
| Scab (Fusarium blight) | 1 .Apply seed treatment.
2. Lengthen rotation (do not plant wheat after corn). 3. Plow under infected residue to hasten decomposition. 4. Do not spread manure that contains corn stalks. |
| Rust (leaf) | 1 .Plant resistant varieties.
2. Plant late. 3. Use foliar fungicides. 4. Apply seed treatment (triadimenol). |
| Scald | 1 .Rotate with crops other than barley.
2. Destroy crop residue by plowing or burning. 3. Plant resistant varieties. |
| Helminthosporium leaf spot | 1 .Plow down residue before planting |
| Wheat soil-borne mosaic virus | 1 .Rotate with crops other than wheat.
2. Plant resistant varieties. |
| Oat soil-borne mosaic virus | 1 .Plant resistant variety (Coker 716 = resistant; Brooks = very suscep.; others = suscep.) |
| Poor plant color | 1 .Good growing conditions or correction of fertility imbalances (not due to infectious diseases) will improve appearance. |