Introduction

Plant diseases can have either abiotic or biotic causes. While abiotic diseases are caused by environmental influences such as temperature extremes, excess or lack of wather or lack or excess of nutrients, the cause of biotic diseases, which are dealt with in this module, lies within pathogens (Greek pathos = suffering, disease). Pathogens are divided into the groups of pseudofungi and fungi, bacteria including phytoplasmas (cell wall-free bacteria) and viruses.

Diseases caused by fungi (Greek myces = fungus) and pseudofungi are called mycoses and pseudomycoses. They are controlled by fungicides (Latin fungus = fungus). Diseases caused by bacteria or phytoplasmas are called bacterioses or phytoplasmoses. They are controlled with bactericides.

Diseases caused by viruses are called viroses. They are controlled with viricides or their vectors (insects, mites, nematodes, fungi) are controlled with suitable products.

In principle, preventive measures for disease avoidance, such as the right choice of location and variety and crop rotation, are the first priority. This can delay or reduce pathogen infection and, ideally, prevent it. In addition, the robustness of the crop can be supported with plant strengthening products by increasing its defenses.

If a disease is suspected, for example in infested areas or in weather conditions that promote diseases, early detection, monitoring and identification of the pathogen are prerequisites for targeted control.

To ensure control success, the right product or product mixture must be used at the right time.

In addition, hygiene measures and forward-looking health management are essential for the following years or the following crop in agriculture.

Way of life of phytopathogenic fungi, bacteria and viruses

In fungi, germinating spores and mycelia (fungal tangles) can colonize and feed on both living and dead host cells. In doing so, the fungus either directly invades epidermal cells or uses the infection route via natural plant openings such as stomata (respiration holes), lenticels (cork warts), hydatodes (water-secreting glands), and wounds. Fruiting bodies with spores formed on the plant surface will usually be spread by wind or rain.

Bacteria enter plants through injuries and wounds. This includes bites or stings from vectors. Bacteria multiply and spread passively throughout the plant within the host tissue or with the sap flow.

Viruses are transmitted mechanically by grafting, injury, and vectors (insects, mites, nematodes, fungi). They enter the cell upon contact with the cell wall and multiply there. Spread within the plant occurs with the sap flow towards the growth zones (shoot tip, root), where uptake by vectors occurs.

Prevention methods for plant protection against diseases in organic agriculture

Learning Outcomes:

• Describe cultural engeneering measures to prevent disease outbreak
• Implement the appropriate ogro technical practice that helps to prevent the outbreak of diseases
• Predict the effect of implementing various agro technical methods on disease progression under specific agro climatic conditions

Choice of location

To ensure the containment of fungal diseases, the choice of microclimate, site and soilis essential. Rapid drying ensures that infection pressure from fungal diseases is kept to a minimum. Wind-open locations are therefore a prerequisite for good air circulation. This is supported by an eastward orientation of sloping sites (vineyards) and an optimization of planting and foliage density of the crop.

The creation of entry points for fungal diseases (viticulture: Erysiphe necator, Plasmopara viticola) through frost cracking is a special issue. Especially due to climate change in recent years, frost locations are becoming the main problem in fruit and wine growing. Flat sites and depressions should be avoided because of the formation of cold lakes. Slopes are less susceptible to frost because the cold can run off. However, shaded sites and north-facing slopes are also gaining in importance. They have an advantage in late frosts due to later bud break. Frost damage can additionally be minimized by using oil products for delayed bud break.

A large number of pathogens in the soil are preserved for many years in the form of permanent spores or similar formations such as sclerotia and microsclerotia (often significantly longer than there are residues of plant tissue in the soil). Nevertheless strict adherence to crop rotation is a prerequisite for disease prevention. When cultivating predominantly non-susceptible species or varieties, diseases with a restricted host range can be "starved out". Some species can only survive as long as at least parts of their host plant are present in the crop. This is usually 1-2 years.

However, special attention is given to non-host-specific pathogens and to disease species that persist in the soil for a long time. Crop breaks of up to 20 years are necessary for persistent diseases, such as Phytophthora cactorum in raspberries and blackberries. In this extreme case, it is recommended to switch from soil to substrate culture, where infected plants can be easily removed from the culture. In the case of infestation with particularly infectious diseases, e.g. in soft fruit (Phytophthora) and in cereals Tilletia caries, long cultivation breaks of 10 years are the rule. In arable farming, sufficient cultivation intervals must be observed, especially for potatoes and legumes, because of soil-borne diseases (4-5 years for potatoes, 5 years for pea and lentil; 3 years for field bean). In addition, in legume cultivation, sufficient intervals to forage legumes or green cover (alfalfa, red clover, sainfoin) must be considered. In orchards, extreme postplanting due to growth depression due to soil fatigue is taking place nowadays. The reason is not sufficiently understood. It is probably a multitude of pathogens. Steaming the cultivated area can remedy this situation!

Soil structure and soil type also have a direct influence on fungal disease infestation. The risk of fungal diseases is particularly high in moist, heavy soils. Cultivation techniques such as ridging or bedding can help by increasing the distance from the crop to the soil, thus promoting warming and drying. Root-borne diseases such as Verticillium, Rhizoctonia and Fusarium occur more frequently in compacted soils. Looser soils are therefore particularly important in horticulture and arable farming. This can be achieved primarily by green manuring. Active loosening is achieved with deep-rooting species (clover, alfalfa, yellow mustard, Phacelia). In viticulture and orchards, perennial greening before cultivation is useful. It is important to note that alfalfa should be avoided in Orchards because of disease transmission of Verticillium and Phytophtora.

Not all diseases have the same growth requirements. For arable and vegetable crops, drought helps well with fungal pressure, but poor water availability negatively affects the crop for bacterial diseases. Loss of turgor can cause wilting to occur more quickly in damaged crops. Avoid intermediate hosts in the immediate vicinity of the crop! Forest edges and windbreak hedgerows pose a higher risk of infection for pathogens without host specificity. For host-specific pathogens, the intermediate host can be specifically avoided e.g.: Juniper for european pear rust.

Choice of variety

In principle, the choice of variety is always dependent on the variety requirements. Extremely important with regard to disease resistance of a variety is the resistance of a plant to biotic and abiotic factors (e.g.: reduced stress to drought, frost, heat and UV radiation) in order to keep the susceptibility to pathogens as low as possible. There are more or less pathogen resistant varieties of all cultivated species. In organic farming, less disease-susceptible, traditional (if possible native) varieties are preferred. In some cases, however, the flavor and yield of the variety are more important than existing resistance. A certain degree of yield loss due to diseases, among other things, is accepted.

The robustness of the plant is defined - in addition to its resistance to abiotic factors - by its ability to repel diseases. The thickness of the epidermis and the wax layer (cuticle) on top of it, as well as strengthening deposits (silicic acid) in the cell walls, play a decisive role here. Thicker-skinned cultivars have an advantage over thin-skinned ones.

While powdery mildew resistance is a priority for all crops, especially in organic farming, additional attention is paid to fungal diseases such as Erysiphe necator and Plasmopara viticola in wine. In organic fruit and wine growing, more and more areas are being planted with new fungus-resistant (PIWI) varieties. In fruit crops, resistant varieties against scab, (Venturia), leaf blotch (Marssonina), fire blight (apple), curl disease (peach), Scharka virus (Plum-pox virus), canker and storage rot (Gloeosporium) and generally disease-resistant raspberry varieties are available.

In arable crops, the focus in resistance breeding is on leaf diseases and Fusarium head blight in cereals, late blight in potato, leaf diseases and corn smut (Ustilago maydis), and Sclerotina, Phomopsis, grey mould rot of the basket in sunflower (Botrytis cineraria).For sugar beets Rhizoctonia-resistant varieties and Cercospora leaf blight susceptible and tolerant cultivars are available.

In horticulture, late blight-resistant varieties in tomatoes and cucumber mosaic-resistant varieties in cucumbers have been bred.

Furthermore, disease pressure from certain pathogens can be circumvented by varieties with suitable planting and harvest dates. For example, early wine varieties are somewhat less likely to be infected late with Botrytis. The likelihood of rain and risk of injury is reduced because the grapes are already harvested in summer.

Choice of rootstocks

(especially against soil-borne pathogens)

Grafting, i.e. grafting a susceptible scion of the desired cultivar, onto a resistant or a robust rootstock (e.g. wild form), increases the resistance of the cultivar. The choice of rootstock balances soil type (lime intolerance of the scion, pH requirements), water demand, vigor and stability, and controls budbreak (early/late). In orchards and vineyards, in particular, less vigorous varieties are desirable because they provide better aeration and thus lower susceptibility to fungi even with less foliage work.

In fruit growing, standard rootstocks are available against various diseases:

• M9 and Genovese in pome fruit against fire blight.
• Docera 6, a hypersensitive rootstock in stone fruit. In plum, enhanced resistance to scab is achieved in combination with scab-resistant varieties.

In addition, in orchards, intermediate grafting is possible with trunk formers to 60-70 cm height. In currant, tall stems are grafted onto Ribes aurorum to obtain upright, stable stems that promote faster drying of leaves and fruit.

The former standard rootstock St. Julian GF6 552, since organic farming and ESFY (European Stone Fruit Yellows) is occurring, is no longer used because it develops stem shoots. This is a disadvantage in the transmission of ESFY by leaf suckers, which suck especially on stem shoots.

If the scion is susceptible to soil-borne pathogens and grows poorly, the use of an insusceptible rootstock variety is recommended (tomato on potato rootstock; cucumber and melon on pumpkin rootstock against Fusarium, Verticillium).

Cultivation measures and soil care

Plant and row spacing are specified depending on the crop and are usually designed to optimize yield. The microclimatic conditions within a crop can be influenced with training systems such as leaf wall management. Soil activation with compost or green manure has a positive effect on the crop. Supplementary irrigation must always be used in a crop-optimized manner.

Foliage management in orchards and vineyards creates a loose plant structure with good ventilation and exposure. While winter pruning establishes the basic shape of permanent crops, summer pruning or thinning reduces leaf mass and stingy shoots. Together, these measures contribute to good aeration and exposure and allow for rapid drying, which minimizes fungal diseases. In principle, the rule is as much leaf mass as necessary, as little leaf mass as possible.

In addition, diseases can be prevented by varying the height of the trunk: for example, the higher the foliage wall starts in the vineyard, the less the splash effect, in which spores of Plasmopara viticola are catapulted by rain from the ground to the lowest layer of foliage. It is also essential to remove stingy shoots on the trunk to prevent P. viticola from "shimmying up" to the foliage zone of the cultivar. In berryfruits, tail systems are preferable to shrub systems.

Soil activation can be accomplished with compost, green manuring, or greening with nitrogen-fixing plants. Generally, greening provides water competition to the crop, but at the same time provides a continuous source of nutrients to optimize plant growth. This leads to an increase in resistance to fungal or bacterial pathogens. A plant overfed by mineral fertilizers - especially nitrogen - is very quickly attacked and damaged by fungi (e.g. Botrytis sp.). Optimally nourished plants, on the other hand, can actively defend themselves against pests and thus resist infestation for longer. Herbaceous plantings with water-saving plants are preferable in planting. Care should be taken not to let the greening become too high because of the microclimatic moisture development and the associated risk of fungal attack. Mowing, rolling or undercutting with "Greenmanager" are possibilities to keep the green cover short. In addition, the greening prevents the spread of pathogens with the soil erosion by wind through its action as an erosion prevention. Supplementary irrigation must necessarily be interrupted during rainfall.

Soil and foliar fertilization

Soil or foliar fertilization of the crop is used to compensate for nutrient deficiencies or imbalances. It can either be applied with in the form of purchased products or stored in the soil as part of the rotation of a previous crop, such as legumes (nitrogen). Excessive or incorrect fertilizer applications can weaken plant health. Nitrogen promotes rapid growth. At the same time, the soft cell walls of the new shoots facilitate the penetration of pathogens.

Plant strengthening

Plant strengthening promotes the robustness of the plant and disease prevention. It may stimulate root growth and support nutrient supply, thus increase stress resistance to environmental factors and promoting healthy plant growth. The use of plant strengthening agents is always preventive. Strengthened plants have strengthened cell walls and epidermis, which prevent or reduce the penetration of pathogens.

For example, the horsetail extract Equisetum plus makes infections by fungal pathogens such as Erysiphe necator more difficult when used regularly, due to the deposition of silicic acid in the cell walls. Plant strengtheners can also activate the plant's own defenses and thus protect against possible infection by microbial pathogens. After their application, there is an increase of phytoalexins (plant defense substances) and so-called ROS defense proteins (reactive oxygen species H2O2; destruction of pathogens invading the plant) in the green parts of the plant. They are responsible for the crop's resistance to disease attack.

Generally, plant and nutrient extracts as well as microorganisms for seedtreatment are used for plant strengthening. Algae extracts possess a high number of micronutrients and increase the tolerance of plant protection products.

Extracts from the following algae species are used for plant strengthening:

• Ascophyllum nodosum (SuperFifty®,AlgoVital Plus®)
• Laminaria (Resistance®)

Encouraging natural enemies and avoid intermediate hosts

In principle, increasing biodiversity in the ecosystem through, for example, flower strips or species-rich revegetation supports the attraction of beneficial insects. These beneficial parasitiods or predators can reduce vectors of pathogens such as aphids or cicadas and thus reduce the likelihood of transmission of viral and bacterial diseases. On the other hand, care should be taken to avoid intermediate hosts of pathogens (e.g.: juniper for european pear rust, Gymnosporangium fuscum).