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Cover crops can reduce diseases and nematodes

Brad Haire strip-till-cotton-planting-georgia-6-a.jpg
Cover crops and conservation tillage may also assist in the suppression of insects, weeds, diseases, and nematodes.

By mid-April 2021, many fields in Tift County, Ga., were tilled and ready for planting to peanuts and cotton. But nothing is ever easy for farmers, is it?  On April 24, nearly six inches of rain fell over the course of a day and night.  

In the aftermath, what had been neatly prepared was now crisscrossed by gullies and precious top soil filled the ditches. Standing beside a farmer as he surveyed the damage, he said to no one in particular, “I’ll plant every acre I own with a cover crop from now on.”

Conservation tillage is an important production practice that brings several benefits. In addition to protecting fields from erosion, planting cover crops can add to soil fertility, reduce soil compaction and, over time, increase organic matter leading to improvements in soil structure, stability, and increased moisture and nutrient holding capacity for plant growth.  Cover crops and conservation tillage may also assist in the suppression of insects, weeds, diseases, and nematodes

“Environment” plays a critical role in the risk a crop faces to disease. Because conservation tillage can improve moisture holding capacity and also reduce soil temperature, the practice affects “environment” and can have an effect on diseases.  Fungal seedling diseases are typically more problematic in cooler and wetter soils. Therefore, growers who use conservation tillage should pay even closer attention to soil temperatures and either delay planting until soil are warm enough or consider using additional fungicides in the furrow or as seed treatments. 

Some winter cover-crops, especially legumes, may also be a nutrient source for fungal pathogens like Rhizoctonia solani. In such cases, it is important that the residue of the winter cover-crop has time to degrade before a susceptible crop is planted. However, other diseases, for example Aspergillus crown rot in peanut, are more severe in hot and dry soils; the benefits of conservation tillage could reduce threat from this disease.

The physical barrier can impact risk to disease as well. Cover crop residue reduces risk of a peanut crop to tomato spotted wilt disease. Conservation tillage reduces risk to leaf spot by delaying and slowing disease onset. Initial infection for some disease comes from residue from the previous year’s crop; rain and irrigation splash surviving spores from the soil to new foliage.  Cover crop residue can help shield from such splash.

Many growers about the benefits of a winter cover crop on management of nematodes.  For producers in the Southeast, interest is often on the impact on root-knot nematodes, southern root-knot nematodes in particular.  There is certainly interest in other nematodes, for example the reniform and soybean cyst nematodes, as well.  Here there are several “main points” to consider.

Choice of a winter cover crop can impact nematode issues later.  Planting a cover crop that is either a non-host or resistant can help to suppress an increase is specific nematode populations versus susceptible weeds or planting a susceptible winter cover crop.  Conversely, planting a cover crop that is susceptible plant parasitic nematodes could result in an increase in population.  For example, as poor/non hosts cereal rye and oats are “good” cover crops to reduce threat of the southern root-knot nematode while wheat and legume cover crops such as alfalfa, hairy vetch, and clover are susceptible and can increase populations of root-knot nematodes. 

Mustard and brassica cover crops (for example rapeseed, turnips, and radishes) produce organic compounds known as glucosinolates.  Glucosinolates degrade resulting in sulfur containing thiocyanates which can act a “biofumigants”. While biofumigants are a “good” thing in the battle with nematodes, diseases, and weeds, their impact is much lower than is the activity of commercial fumigants. The biotoxic activity from mustards and brassicas activity also varies depending on species, planting date, growth stage when killed, climate, tillage system and time until the next crop is planted.

Climate during the winter plays an important role when anticipating risk to plant parasitic nematodes in the spring.  Soil temperatures below 60°F help to suppress growth, development, feeding, and reproduction of plant-parasitic nematodes, regardless of cover crop that is planted.  However, in La Niña winters like we are experiencing now where soils were slow to cool and could warm quickly, nematodes may build rapidly on partially susceptible cover crops, weeds, and cotton stalks and other “volunteers” that survived the from 2021.

Cover crops and conservation tillage have many important benefits for row crop production in the Southeast, not the least of which is to help mitigate soil erosion.  Use of cover crops can also be beneficial for disease and nematode management, however careful planning is required to ensure that one’s expectations from a cover crop are realistic.

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