Soil Biology: Cover Crops and Disease Suppression

Kathryne Everts | 2016

Introduction

Cover crops provide several benefits to soil health such as improving soil structure, reducing the need for synthetic chemicals by decreasing weed biomass, increasing soil organic matter, contributing nutrients to the soil, retaining soil moisture, and decreasing soil erosion. In addition, the integration of cover crops into crop production often leads to soils that are suppressive to plant diseases (i.e. have less potential for disease development). Disease reductions may occur in fields where the cover crop is planted in the fall and tilled under in the spring as a green manure prior to planting the cash crop, as well as when the cover crop is killed and the residue is left on the soil surface as a mulch.

Mechanisms of Disease Suppression

Mechanisms of cover crop-induced disease suppression are not yet fully understood. However, several common mechanisms are thought to be involved in the “general suppression” of soil-borne plant diseases. Disease may be suppressed due to an increase in the overall activity and diversity of the soil microbiota (microorganisms that inhabit the soil) that occurs with cover crop production. Greater microbial diversity and activity results in increased competition with plant pathogens for nutrients and release of more compounds that interfere with the ability of plant pathogens to grow and develop. Some cover crops impact plant pathogens directly by releasing fungitoxic compounds (compounds that are toxic and subsequently unfavorable to the growth of fungi). In addition to increasing competition with soil-borne plant pathogens, these changes may also impact diseases because decomposing organic matter may increase fungistasis. Fungistasis occurs when a soil-borne plant pathogen’s growth and infection is inhibited, even under optimal soil conditions. Fungistasis results from the presence of volatile compounds and/or the reduction in organic carbon compounds and nutrients. One example of a cover crop that may trigger several of these impacts is mustard greens (Brassica juncea). Mustard greens contain high levels of glucosinulates, which are sulfur containing chemicals that have fungicidal and nematicidal properties. The glucosinulates in mustard greens induce high levels of biological activity (mostly antimicrobial) and successfully suppress the occurrence of Rhizoctonia on potatoes through the release of isothiocyanates into the soil. In addition to direct effects on plant pathogens, many cover crops impact plant pathogens indirectly by triggering the plants’ host defense response (a plant’s immune response that protects it from infection).

In addition to general suppression, cover crops may also induce specific suppression by enhancing individual beneficial organisms. An example of an organism that induces specific suppression is the fungus Trichoderma harzianum, which suppresses Pythium, Fusarium spp. and other soil-borne pathogens of beans, and many other vegetable crops. Suppression by T. harzianum is thought to be due to competition for nutrients. Trichoderma is able to colonize many cover crops including annual ryegrass (Lolium multiflorum), red clover (Trifolium pretense), hairy vetch (Vicia villosa), and winter wheat (Triticum aestivum). Its ability to survive at high populations and colonize a subsequent cash crop is related to the cover crop root mass, time of cover crop termination, and other factors. In one study, winter wheat and canola (Brassica napus) resulted in the best carry over of Trichoderma. Due to its ability to reduce some diseases, T. harzianum has been formulated into a commercial biocontrol product.

Mycorrhizae (fungi that live in association with plant roots and benefit the plant by aiding in water and nutrient absorption) may suppress individual pathogens. Cover crops influence the quantity and composition of mycorrhizae in soils and on the subsequent cash crop. Investigators have observed enhanced mycorrhizal populations in peach and tomato following a mycorrhizal cover crop. More recently, there was an increase in mycorrhizal colonization in watermelon grown after a hairy vetch or crimson clover (Trifolium incarnatum) cover crop. The cover crops improved mycorrhizal colonization of the watermelon roots and also reduced Fusarium wilt.

No-till and Disease Suppression

No-till cover crops provide many of the benefits just described and, additionally, provide a physical barrier that reduces the splash of soil and soil-borne pathogens onto foliage, stems, or fruit. The cover crop can also reduce the presence of free moisture on the plant because they reduce soil splash. Septoria leaf spot severity was reduced on tomatoes grown in a hairy vetch cover crop mulch due to reduced soil splash to the tomato leaves. Foliar and fruit rot diseases of pumpkin such as white fleck (caused by Plectosporium tabacinum) and black rot (caused by Didymella bryoniae) are often lower when the crop is grown on a no-till hairy vetch, cereal rye (Secale cereale), or hairy vetch plus cereal rye cover crop. The reduction may be due to the formation of a cover crop vegetative layer between the fruit and soil reducing soil splash and shortening the length of time the fruit remain wet throughout the day.

Cover Crop Management

Selection of a cover crop depends on many factors including its ability to suppress disease. In considering a cover crop for disease suppression, consider field history and what diseases have been observed in the past. In addition, consider future crops and their potential pathogens.

Cover crops, even those commonly associated with disease suppression, can under some circumstances increase other diseases. In addition, timely incorporation of a cover crop is very important because incorporation that occurs too close to planting the cash crop may increase pathogens such as Pythium spp. It is important to maximize biomass of the cover crop, however the cover crop must be incorporated to insure enough time to breakdown, usually several weeks, prior to planting. Like all plants, cover crops get diseases and therefore can host plant pathogens, increasing the population present on the subsequent cash crop. For example, hairy vetch is a host of root knot nematode. White clover (Trifolium repens) and buckwheat (Fagopyrum esculentum) cover crops increased bean root rot where Fusarium, Pythium, and Rhizoctonia were present. Though brassica cover crops suppress many diseases, there also are reports of an increase in Fusarium disease severity following brassica cover crop incorporation. This increase may have resulted because a Brassica cover crop decreases the mycorrhizal colonization of the succeeding cash crop. For example, when tomato was planted after a mustard (Allaria petiolata) cover crop the tomato roots had lower mycorrhizal colonization than in the absence of the cover crop.

The use of cover crop mixtures, timely incorporation of the cover crop into the soil or selection of a different cover crop can minimize these problems.

This product was developed with support from the Southern Sustainable Agriculture Research and Education (Southern SARE) program, which is funded by the U.S. Department of Agriculture—National Institute of Food and Agriculture (USDA-NIFA). Any opinions, findings, conclusions or recommendations expressed within do not necessarily reflect the view of the Southern SARE program or the U.S. Department of Agriculture. USDA is an equal opportunity provider and employer.