Cropping Sequences and Root Health

See also:

• Assessing Fungal and Nematode Root Pathogens With Visual On-Farm Soil Bioassays
• Cornell Soil Health Testing Laboratory
• Bean Root Bioassay Demonstration
• Biofumigation for Managing Soil-Borne Pathogens

Root health is a good indicator of overall soil quality and health, as roots are influenced greatly by the physical, chemical and biological properties of soil. Roots growing in a healthy soil are generally large, coarse, firm, and white in color. They penetrate deeper into the soil, have a large number of fine fibrous rootlets and exhibit limited to no symptoms of damage from root pathogens. Obviously, such roots are highly efficient in absorbing water and nutrients and are more tolerant of damage caused by pathogenic organisms and unfavorable environmental conditions, thus contributing to overall crop health and productivity.

The two principal decisions made by growers for any target field is first selecting both the main/cash and cover crops(s) then second determining their order over the course of the production cycle. The latter decision will have a profound effect on overall soil health, root health and yield potential of the various crops selected. The effect of differences between cropping sequences on soil and root health is partly due to the growth habit of the roots of the selected crops, the quality and quantity of organic matter returned to the soil, the degree of surface cover, and the susceptibility of the crop to root pathogens and other pests as well as their impact on the total soil microbial community and diversity. Below is a brief discussion that includes examples to convey the influence of cropping sequences on root health as impacted by pathogenic fungi and plant-parasitic nematodes.

Traditionally, crop rotations have been designed to facilitate the shortest possible rotation for managing pests and achieving acceptable crop yields. This is possible if a proper cropping sequence (main and cover crops) is practiced early, before the soils of the target fields become deteriorated and/or heavily infested with root pathogens and other pests. Crop rotation is one of the oldest and most effective methods for managing plant diseases. When a pathogen population becomes large and damaging, planting a non-host crop, antagonistic crop, and/or a resistant variety can greatly reduce the population size and damage of the pathogen. Designing a proper cropping sequence of main and cover crops to maintain root health is not possible without the accurate diagnosis of the prevailing or potential root pathogen(s) in the fields of interest. Much can be learned about the incidence and damage of root pathogens by carefully digging out and examining roots for symptoms and signs of root diseases or periodically conducting an on-farm, visual soil bioassays with beans, lettuce and soybeans to assess for soilborne fungal pathogens, root-knot and lesion nematodes, respectively. See:

• A Soil Bioassay for the Visual Assessment of Soil Infestations of Lesion Nematode
• Visual Assessment of Root-Knot Nematode Soil Infestation Levels using a Lettuce Bioassay

Obviously, knowledge of the susceptibility of the crops included in the entire cropping sequence to the identified pathogen(s) is critical. Some crops, although not severely damaged by pathogens themselves, can serve as good maintenance hosts and enable the pathogen population to build-up while others are non-hosts and can reduce the population of a particular pathogen. This concept is fairly straight forward when managing for one root pathogen. However, the presence of two or more pathogens can make it more challenging to design an effective cropping sequence and hence may require the use of other management options including available chemical or biological control products.

A common rule of thumb in designing cropping sequences is to rotate by crop families or to avoid planting closely related crops. Unfortunately, this is not always effective as many of the root pathogens such as Rhizoctonia, Pythium, Meloidogyne, and Pratylenchus that are prevalent in NY and the NE region have a wide host range and damage plants in numerous crop families including the Umbelliferae/Apiaceae (carrot), Liliaceae (onion), Fabaceae (bean), Brassicaceae/ Cruciferae (cabbage), Solanaceae (potato), etc. In addition, several locally adapted legumes including soybean, clovers, alfalfa, and hairy vetch that are known for their numerous attributes to help improve soil health properties such as soil aggregation, water infiltration, and nutrient cycling/ nitrogen fixation, etc., are also highly susceptible to the same four pathogens. On the other hand, grain crops are good rotational crops for reducing the major root pathogens of vegetables, except the lesion nematode. Also, there are a number of antagonistic crops such as sudangrass and sorghum-sudangrass hybrids, several cruciferous crops, and selected cyanogenic white clover and flax varieties that when added as green manures into warm soils can function as biofumigants and reduce populations of root pathogens. Biofumigant mustard cover crops have become available and proven effective since this article was first written.

We have evaluated the effect of green manures of several crop species on the population of lesion nematode (Pratylenchus penetrans) in roots of snap beans grown subsequently in the amended soil.  The number of lesion nematode per gram of bean roots was highest after hairy vetch and alfalfa, but lowest after rapeseed, sudangrass, ryegrass, and mustard (Table 1). In a greenhouse trial, similar results were found on the root health of snap beans grown following the incorporation of select cover crops as green manures in our root rot field soil which was heavily infested with the soilborne pathogens Rhizoctonia, Pythium, Rhizoctonia and Fusarium. The green manures differed in their ability to suppress root disease development on the hypocotyl and roots of the subsequently grown snap beans. The snap bean roots appeared the healthiest following the rapeseed and crown vetch green manure crops (Table 2). Table 3 documents the effectiveness of corn in a vegetable rotation on muck soils in controlling the root-knot nematode and its damage to susceptible crops such as onion and carrots. The effect of one-year rotation or longer with corn on increasing snap bean yield and reducing root rot severity is illustrated in Table 4.

Table 1.  The effect of selected cover crops when used as a green manure on the lesion nematode population (Pp) in the roots of a subsequently planted snap bean crop in the greenhouse.

Table 2.  The effect of selected cover crops when used as a green manure on the root health in the roots of a subsequently planted snap bean crop in the greenhouse.

* Root health was rated on a scale of 1 to 9 based on the health of the hypocotyl and roots. A rating of 1 = white coarse textured hypocotyl and roots; healthy while a rating of 9 = ≥ 75% of the hypocotyl and roots are severely symptomatic and at advanced stages of decay.

Table 3.  The effect of crop rotation on the root-knot nematode infestation levels in three vegetable fields as determined using a soil bioassay with lettuce in the fall 2004, 2005 and 2006 shortly after harvest.

* Root-galling severity was rated on a 1 to 9 scale with 1 = no galls (healthy roots) to 9 (>80% of the root system is galled).

** Medians followed by different letters within rows are significantly different based on a nonparametric repeated measures analysis.

Table 4.  The effect of different corn and snap bean cropping rotations on plant emergence, stand establishment, root health, and yield of snap bean.

* In the field at flowering, the roots of 20 plants were rated for root health on a scale of 1 to 9 based on the severity of root disease symptoms on the hypocotyl and roots.

Prepared by/more information:

Originally pepared by George S. Abawi, Beth K. Gugino, and John W. Ludwig for Vegetable MD Online, March 2008. Updated by Margaret McGrath March 2021.

For more information, contact:

Beth Krueger Gugino
Professor in Vegetable Crop Pathology
Department of Plant Pathology and Environmental Microbiology
College of Agricultural Sciences
The Pennsylvania State University
219 Buckhout Laboratory
University Park, PA 16802-4507
bkgugino@psu.edu