Does introducing legumes into cover crops pose a health risk for protein crops ?

In the seventh episode of the Radio Carbone podcast, we look at the health risks associated with the use of legumes as intercropping cover crops in rotations that already include protein crops. The inclusion of legumes in rotations, in the form of cash crops (protein peas, faba beans, alfalfa, etc.) or service crops (associated plants, intercropping cover crops) offers a number of advantages, such as the end of the rotation, symbiotic nitrogen fixation and the development of mycorrhizae. Particular care must be taken when using certain species in cover crops, so as not to encourage the emergence or maintenance of certain diseases affecting cash crops.

Question from Alain in Charente-Maritime

Alain, a farmer in Charente-Maritime (Nouvelle-Aquitaine, France), is involved in the Soil Capital carbon programme. He is wondering whether increasing the proportion of legumes in his plant cover could pose a health risk for his protein crops (18% of his UAA in 2022).

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‍Icosytème's recommendations for legumes

‍Gilles Sauzet responds to Alain by stressing the importance of knowing the infectious potential of your soil and choosing the species and varieties you use. According to the expert, attention must be paid to two fungal diseases in particular: aphanomyces and botrytis.

Aphanomyces

Aphanomyces (Aphanomyces euteiches) is a root rot affecting legumes such as lentils and peas. The symptoms are chlorosis of the leaves and, more generally, degeneration of the plants, which can lead to major losses.

Gilles starts by reminding us that rotation is the first lever for preserving the health of the soil: while a minimum of 5 years is often recommended before returning to the same plot with a sensitive protein crop (see box), it is advisable to extend this return period to 7 years if sensitive species are also included in the cover crops. Gilles also strongly recommends carrying out a soil test to determine the infectious potential of the plots. Infectious potential (IP) scores vary from 0 to 5:

• PI < 1: plot with little or no infestation (pathogen not detected overall)

Legumes can be used as cover crops or intercrops, although resistant species (or varieties) should be favoured because of the presence of susceptible protein crops in the rotation.

• 1 < PI < 2.5: moderately infested plot

Damage can occur on spring protein crops such as peas and lentils. Resistant to very resistant species should therefore be chosen for combined cover crops or intercropping.

• PI > 2.5: heavily infested plot (proven presence of inoculum)

Presence of non-resistant legumes totally inadvisable. If legumes are used in association or intercropping, opt for species such as faba bean, lupin or fenugreek.

In the absence of a soil test, Gilles recommends using only resistant to very resistant species/varieties.

Classification of several legume species according to their susceptibility to aphanomyces

• Susceptible species (host plants that multiply the pathogen): peas, lentils, alfalfa, certain varieties of clover, vetches, vetch, green and kidney beansNon-host species: chickpeas, lupins, trefoil, fenugreek
• Host species but resistant: faba beans, soya and sainfoin
• Susceptible species with resistant varieties: Alexandria clover (var. Tabor), crimson clover, white clover, common vetch.

See Terre Inovia and Arvalis publication (2017)

Botrytis

Botrytis (Botrytis cinerea) is an aerial disease that mainly affects faba beans. Affected plants show numerous small brown spots. The disease develops in damp, mild weather, and the spores are carried by the wind.

For Gilles, the main risk is posed by early sown and emerged faba beans, for example in early summer in intercropping cover crops, which can contaminate a faba bean intended for harvesting in a neighbouring plot in winter. The risk is increased when field beans are sown early and at a high density. If the situation arises, Gilles recommends destroying the cover crop early.

Conclusion

The presence of certain legumes in intercropping or as associated plants can contribute to the maintenance of pathogens affecting protein crops, in particular aphanomyces. However, provided that suitable species and varieties are chosen, and the infectious potential of the soil is monitored (as far as possible), the risk can be greatly limited or even eliminated. Disease risk is therefore a criterion for considering, rather than excluding, the use of legumes as cover crops in cropping systems that include protein crops.