Are you scouting your forage crops for disease this fall? This may not be the first item on a producer’s to-do list, but it’s important.
In order to have disease occur, a pathogen, a susceptible host, and favorable conditions need to be present. Not all plant problems are linked to pathogens, which is all the more reason to avoid automatically applying pesticides.
Many problems are often linked to nutrient availability and uptake (don’t guess, soil test!), drought or flooding, light availability, soil compaction, insect damage, etc.
Pastures that are stressed can be more at risk for pathogens to invade. For example, poor soil drainage provides adequate conditions for soil-borne pathogens like Phytopthora root rot to infect plants. In these cases, the stress needs to be removed in order to improve the health of the plant.
Getting it started
Diseases can often crop up during forage establishment. During germination and emergence, plants are more susceptible to damping-off, a condition that generally occurs in excessively wet conditions and results in collapse and death of seedlings.
Legumes, in particular, tend to be more susceptible during the initial year after planting.
Fall is a great time for seedlings to become established in a pasture, and it is, therefore, a good time to keep an eye on seedling emergence.
Did seed fail to germinate?
Seed viability, seedbed conditions, and the presence of herbicide residue may be the cause rather than a particular pathogen. Did the seed germinate but not emerge or did the seedling emerge but not survive?
While planting depth, compaction, and environmental conditions can play a role, a pathogen could very well be the culprit.
Legume diseases
Legumes have many diseases that affect yields. Common root and seedling issues in alfalfa, for example, include bacterial wilt and Phytophthora root rot. These result in damping-off symptoms, yellowing, decaying roots, and stunting. Often, these diseases are related to poor soil drainage.
Diseases such as anthracnose, lepto leaf spot, black stem, and verticillium wilt are also present in this crop and can appear during the fall months. Check the lower leaves of alfalfa plants as this is usually where symptoms will first occur.
Other legumes, including clovers, are susceptible to similar diseases. Fusarium root rot and Cercospora affect the crowns and roots of legumes, while symptoms of diseases such as powdery mildew and mosaic viruses will typically appear on leaves.
Management
Management of diseases in legumes will often depend on good management practices for that crop. Avoid planting in poorly drained fields when possible, use a soil test to check soil pH and fertility needs, use crop rotations in pure stands of alfalfa, make sure equipment is clean, mow younger stands first and after dew has dried, and take cuttings at the right heights.
With some leaf spot pathogens, mowing earlier in the season before leaves fall off may prevent significant yield loss.
Many issues with diseases can be resolved by planting resistant varieties. For example, many varieties of legumes sold today are resistant to wilts.
Grasses
Both warm and cool season grasses are susceptible to a variety of pathogens as well.
Ever walked through the field and notice your shoes turning orange? This is a good indicator plants in your field are infected with rust, a group of fungi that can reduce yields, quantity of seeds produced, forage quality, and palatability.
Another disease is brown stripe of orchardgrass, which can cause tip dieback in stands and impact nutrient value of the grass. There are currently some varieties of orchardgrass available that are resistant to brown stripe.
And the list of diseases goes on.
Control, prevention
Control of the aforementioned relies on avoiding excessive nitrogen application (some diseases have been linked to over application of fertilizers), seeding fields with a diverse seed mixture, using good grazing practices, and using resistant cultivars and only good quality, certified seed.
Risk to livestock
A few plant pathogens, particularly those belonging to the fungi group, may cause illness in the livestock that consume them. Many fungi produce mycotoxins that can vary in toxicity depending on the concentration, exposure, and environment the fungus developed in.
Deoxynivalenol and zearalenone, which are produced by Fusarium fungi, are a couple of examples of mycotoxins that can affect livestock digestion and reproduction.
Commercial mycotoxin tests are available to check for presence and concentrations of these substances as well as other mycotoxins in feed.
Using a good nutrition program that includes adequate amounts of protein, effective fiber, and antioxidants (vitamin E, selenium) as well as reducing stress for the animals can help mitigate the effects of some mycotoxins.
Other management strategies for mycotoxin-contaminated forages will depend on the pathogen and host present.
The fungal pathogen, Rhizoctonia leguminicola (now Slafractonia leguminicola), that causes black patch in clover and alfalfa produces the alkaloid slaframine. Slaframine is known to cause excessive salivation (slobbers) and reduced weight gain in equine. The fungus also produces swainsonine, which has been linked to neurological issues in livestock.
Management of black patch involves separating the animal from the contaminated feed source — this may include removing hay containing infected plants and applying a general broadleaf herbicide to kill infected clovers and alfalfa in the field.
When the disease cycle is broken by the removal of the host and when brown spots no longer appear on the leaves of legumes that have since grown in the field, these forages are safe to graze again.
A different strategy is used to control another alkaloid-producing pathogen that is well known — ergot. While ergot is mostly associated with rye, the fungus can also infect many other grass species found in pastures.
Symptoms of ergotism in cattle can include a rough coat of hair, vasoconstriction, lameness, weight loss, necrosis in the extremities, and extended stays in the shade or water.
Inspecting seed heads for thin, elongated black structures, known as sclerotia, is important in identification. Mowing fields before grass goes to seed and removing seed heads by clipping before grazing can reduce the risk of ergotism occurring in livestock.
Keep watch
This is by no means a full list of diseases found in forage crops or the strategies used for controlling them. We always recommend an integrated pest management (IPM) approach for disease control in crops.
The first step in IPM is accurate diagnosis — be 100% sure of what is going on in the field. Knowing what is causing plant disease and the life cycle of that disease-causing organism will increase the effectiveness of disease control. Control strategies will be based on the organism identified, site conditions, costs, and weather predictions.
Know what your fields look like in a good year — a glance is not enough to get an idea of whether or not there is a disease problem that warrants treatment.
Scouting needs to occur regularly throughout the growing season. Use a GPS unit to mark the location of suspect plants and take pictures or drop off samples at your local Extension office for identification and site-specific management recommendations. Keep in mind the presence of a disease could be a symptom of a much larger issue within a field.
