Enhancing crop resilience and resistance

Learning Outcomes:

• Discuss the importance of enhancing crop resilience and resistance in organic plant protection.
• Explain the influence of site and cultivar selection and crop planning on the prevention of pest outbreak.
• Describe soil and plant management practices favourable for the regulation of pest population in organic farming.

Site selection

Site selection for the crop establishment is very important for the economic success of any plant production. In the organic farming, even more attention is given to the site selection because it can considerably influence the development of diseases, pests and weeds. Generally, it can be said that organic fields should be established on the best cultivation sites for a particular type of production. Appropriate topography, such as flat terrain for vegetables, or hills and slopes of suitable exposure for vineyards will ensure good lighting and airiness. In such conditions, after precipitation, the vegetative mass dries quickly, so the conditions for the development of fungal diseases are unfavourable. The soil on which organic crops are grown should be moderately fertile and well drained with a high content of organic matter. In this way, the vigour of plants will be moderate, and thus the risk of fungal diseases. Favourable soil conditions are important for the development of crop roots, but also for increasing the diversity of the population of useful microorganisms and other animal species which help to regulate the population of weeds and soil-borne diseases. It is important to consider the natural vegetation or agricultural areas around the future field as well as the vegetation on the future field itself since they can be a source of disease or host plants to problematic pests and disease vectors. Moreover, it is necessary to avoid cultivating near abandoned fields. It is recommended to establish crops in areas where the diversity of agricultural crops is wide and agricultural areas are combined with natural habitats in order to create a more active ecosystem.

Crop planning and crop rotation

The organization of crop rotation, i.e., spatial and temporal change of crops is an inevitable measure in the production of arable and horticultural crops. It has a great importance in organic production since it is a fundamental measure for pest regulation. It is an ancient human experience that long-term cultivation of the same crop accumulates diseases, pests and weeds in the soil, and thus this was the reason for the crop rotation introduction.

Consecutive cultivation of the same culture affects the structure of microorganisms’ population in the soil, i.e., it causes a decrease in the number of useful microorganisms and fauna, and the spread of pathogens in the soil. Although soil diseases are slowly transmitted and are initially limited to smaller areas and a smaller number of infected plants, by growing the same or related crops on the same land, the number of pathogens and infected plants will increase from year to year. A particular problem is the accumulation of parasitic nematodes and virus-vector nematodes in the soil. Some crops, such as potatoes, are particularly susceptible to nematodes, while the nematodes are virus vectors in others, such as grapevine. The most successful way to control nematodes is to change crops, grow resistant varieties and destroy their host weeds.

In continual cultivation, weed companions are widespread. Thus, it is required to alternately shift monocotyledons with dicotyledons, narrow spacing crops with wider spacing ones, sowing of broadleaf species after narrow leaf ones, etc.

When compiling the crop rotation, it is necessary to know the characteristics of each individual species, their tolerance to repeated cultivation as well as their interrelationships. It is mandatory to alternate non-related species with diverse growing requirements and characteristics, such as cereals, vegetables and root species, and to avoid the cultivation of related (potato/tomato, celery/carrot) species one after another. Cereals can be grown more often in crop rotation because they are not conducive to the development of diseases in the soil, while the crops that are susceptible to soil diseases should be planted in crop rotation rarely or always on a new surface. By alternating species that a particular pest feeds on with those that a pest does not eat is a long-term strategy to reduce their population.

A well-designed crop rotation will reduce the accumulation of weed seeds in the soil but will also reduce the appearance of new seeds. It is recommended to alternately grow fast developing species that provide high planting density with species that can be dug for a long time. If the population of perennial weeds grows despite all the measures taken, crop rotation is one of the few opportunities to reduce them.

Simultaneous cultivation of two or more crops on the same lot (consociation) has many positive crop rotation characteristics since it provides optimal use of available space in the field and helps with pest prevention. It can be organized in different ways, from sowing two or more species together randomly on the same surface, through sowing intercrops of one species in the interrow spacing of another, to alternating several rows of one species with several rows of another one. The growth of plants in consociation stimulates a rich and diverse life in the soil and thus helps to control both harmful organisms in the soil and weeds. The different species in the field provide a fast-growing and well-covering vegetation layer that prevents weed development. If another crop is grown in addition to a crop with a large space between plants, the vegetation layer on the soil will develop faster and less effort will be needed to regulate the weeds. For example, if tall crops that ripen earlier are grown with those that remain low to the ground and ripen later, the growth of the second crop will be slower at first, but after the end of a high crop growing season, the second crop will start growing more intensively and thus prevent weed growth.

The second crop can be used as the vegetation around the field to serve as a barrier to fungal spores, pests and virus vectors. Furthermore, some species may attract natural enemies or act as repellents for pests of the species with which they are grown in consociation.

Cultivar selection, seed and planting material

Given the narrow range of permitted measures and plant protection products in organic farming, one of the most effective strategies to combat diseases and pests is the cultivation of resistant varieties. Of course, there are no resistant varieties of all species nor those that would be resistant to all pathogens of one species. However, resistant varieties should be sown/planted whenever possible, and they should be preferred even at the cost of compromising with some other important economic characteristics. There are two main reasons for this. Less susceptible varieties are less likely to be infected than susceptible ones and can be grown in the presence of certain pathogens without major damage. The population of pathogens will be reduced by their cultivation, which will allow the cultivation of somewhat more susceptible varieties after a few vegetations. It is also appropriate to simultaneously grow more varieties of different resistance, which is closer to the traditional method of cultivation where the planting material was not so genetically uniform. In this way, less susceptible plants will not develop symptoms or will have fewer symptoms, and part of the spores will retain on them instead of susceptible plants. However, the cultivation of resistant varieties leads to the adaptation of pathogens by the development of new strains that can overcome resistance. Therefore, growing resistant and less resistant varieties together will slow down the appearance of such strains.

One of the strategic controls for diseases and pests in the soil is grafting on resistant rootstocks, for which a good example is the grafting of grapevine on Phylloxera-resistant rootstock.

Planting/sowing healthy planting material is one of the standard phytosanitary measures in modern agriculture. It has additional importance in organic production. Infected planting material brings the source of infection and weed seeds into the crop plantation and allows the development of diseases and weeds early in the growing season when the young plants are especially susceptible which can cause serious damage. These pests are regularly well adapted to a particular species, while some new diseases, pests or weeds that were not present before and to which the producers are not accustomed can be introduced to the farm through infected material.

Soil management

Fertile soil is of the principal value of every plant production. In conventional agriculture, favourable conditions for the growth and development of cultivated plants are created by intensive mechanical tillage and by the addition of easily available nutrients in the form of mineral fertilizers. Organic production is based on a completely different paradigm. Here, the soil is considered as a living organism and all measures are directed toward creating favourable conditions for the development of diverse microorganisms and fauna in the soil that will provide the necessary nutrients for cultivated plants through complex processes of organic matter production and decomposition.

Fertile and well-structured soils will provide optimal conditions for plant growth, which increases their resistance to pests. It is important to fertilize in a balanced way to ensure a sufficient amount of P and K, while N should not be excessive. Overabundant amount of N makes the crop tastier for insects. It causes high plant density and robust vigour. High humidity provides favourable conditions for the development of the disease. Moreover, it is demanding to monitor the diseases and pests’ symptoms and to apply plant protection products in such conditions, while it is harder for natural enemies to find pests. Therefore, in organic farming, manure is applied almost exclusively with organic fertilizers (stable manure, compost produced on one's own farm, etc.), which are gradually mineralized and mineral nutrients are released from them. Fertilization with organic manures ensures the maintaining and increasing of humus content, which is essential for fertility and microbiological activity of the soil.

Increasing the diversity of species in the soil is an important task in organic production because through their impact on nutrient circulation some of the soil microorganisms, natural enemies or small animals in the soil directly attack pests and destroy weed seeds. Tillage, and in particular the mixing of soil horizons, is reduced to a minimum to provide the conditions for soil organisms to be as favourable as possible. Wherever possible, the soil is maintained by cover cropping or mulching, thus creating a layer of soil with a crumbly structure, rich in organic matter and soil organisms.

Diverse cover crops are also habitats for natural enemies living above the soil. Cover cropping with fast-growing species that cover the soil is one of the most successful strategies for weed control especially in fields where crops are grown with large inter row distances.

Cover cropping with annual species (green manuring) can also be applied between harvesting and the beginning of new growing season/planting of new permanent crops. This is a good way to maintain the soil in areas where due to the small amount of precipitation is not possible to permanently grow cover crops together with perennial crops. Selection of proper species for green manuring can reduce the pest population left over from the previous crop, prevent weed growth and nutrient leaching in the soil. Their ploughing brings in fresh organic matter used to feed on microorganisms and other beneficial organisms in the soil.

Many of the benefits of crop covering, particularly the impact on the soil biodiversity improvement, are also achieved by mulching. In organic production, mulching with organic materials is applied, most often with straw or freshly cut grass. That kind of covering significantly affects the weed development, making it difficult for them to grow through the layer of organic matter and preventing the light necessary for germination.

Appropriate irrigation method can also affect the diseases development and should be adapted to specific crops. The amount of water in one round of watering, the irrigation frequency and technique can affect the spread of disease and the severity of the damage they cause. For example, if furrows are irrigated, it is suitable to water them more often with smaller amounts of water, while the sprinkler irrigation system is better to operate late in the evening or at night when dew is already forming. Considering disease prevention, the most suitable is localized irrigation (drip irrigation) where small amounts of water are applied to the plant root, while the aboveground plant organs are not moistened.

Plant management

The development of diseases and pests, their monitoring and the application of plant protection products are significantly influenced by the various plant management measures that are carried out on the plants we grow. Different interventions are carried out on different crops, yet they should provide balanced vegetative and generative growth for all crops. Thus, during certain critical periods of development, vigour is reduced, while better nutrition of fruits is provided, which increases their fertility and quality. In addition, by removing redundant vegetative organs (such as laterals, tops of shoots, leaves in the fruit zone or shoots from the trunk) ventilation and quick drying take place which create unfavourable conditions for the fungal diseases’ development.

Winter pruning is regularly carried out in permanent crops such as vineyards and orchards, which has a similar purpose as the interventions carried out during the growing season. When performing winter pruning, from the phytosanitary point of view, it is important to leave only the shoots without symptoms of diseases and remove all unnecessary parts where pests can overwinter. It is not appropriate to chop the pruning residues and leave them on the ground as mulch or plow them into the soil since they can be a source of infection in the next vegetation.

It is required to remove all the shoots discarded by pruning from fields with perennial crops, and to remove all leftovers after harvesting annual crops since they can be an infection source in the next growing season. This is especially important for highly infectious pathogens that develop early in the growing season from the debris of the previous one. Composting plant debris will ensure the circulation of nutrients within the farm. During the production of compost, a high temperature develops which destroys pests, thus the obtained organic fertilizer can be incorporated into the soil without the danger of spreading the infection.

It is recommended to continuously remove infected plant parts while the level of infection is still low during the growing season.