Widely used in cooking, in animal nutrition and in medicine for its aromatic and therapeutic properties, the onioncan't miss in a self-respecting garden. The various layers that characterize the bulb of this biennial herbaceous species, grown as an annual, of the Liliacee family are not sufficient to protect the plant from pathogens (virus, bacteria is mushrooms). If you opt for the organic cultivation of the onion, you have to learn to recognize adversities and to fight them, but above all to prevent them with the right precautions.
Onions can be attacked by mosaic or yellow mosaic viruses and by bacteria that cause three types of rot (soft, sulphurous and acidic) during harvesting and storage. There are several mycosis caused by fungi during cultivation which, together with the attacks of the parasites, negatively affect the productivity of the horticultural species. Downy mildew, botrytis, alternariosis and charcoal occur in the aerial part of the onion, while fusarium, anthracnose, pink root rot and carbonaceous rot are found in the hypogeal part.
Let's find out the three most frequent fungal diseases of this crop, which can be defended from attacks with natural methods.
Downy mildew of the onion
While resembling a mushroom, the downy mildew (Downy mildew schleideni) belongs to the kingdom of the Protists, to the Peronosporacee family. When it attacks the onion, oomycete gives rise to elongated and whitish spots on the leaves that turn into a gray-purple mold in conditions of high relative humidity, while with low humidity levels they tend to necrosis. The affected leaves turn yellow and curl downward manifesting rot, while the bulbs are small and generate stunted seedlings.
Since downy mildew requires the presence of water to develop, it is essential to avoid water stagnation by draining excess water and carrying out a good working of the garden soil. Other useful practices to prevent the disease are the use of healthy bulbs or resistant cultivars, crop changes, the destruction of plant residues that could be infected. In addition, when fertilizing the onion it is imperative to pay attention to the nitrogen supply, which must not be excessive. To combat the pathogen it is also possible to resort to horsetail or dandelion preparations and, where strictly necessary, to use copper-based products.
The horsetail preparation (Equisetum arvense) is a decoction of the leaves and stems of the plant, which grows spontaneously on wet uncultivated land. The decoction is applied on onion leaves once every 15 days and has an anti-fungal action by strengthening the culture against downy mildew. Filtered and diluted 1:10, the preparation is also used as a foliar or root fertilizer and can be mixed with nettle. The dandelion preparation (Taraxacum officinale) is a decoction of the whole plant that increases the resistance of the onion to disease.
THE cupric products most used against downy mildew are the copper oxychlorides and the Bordeaux mixture. Tetraramic oxychloride is a substance with a ready effect, while copper and calcium oxychloride is more effective, but also more persistent than the tetraramic. The Bordeaux mixture has a variable activity depending on the ratio between the copper sulphate and the lime. Treatments with copper-based products are necessary when the climatic conditions are particularly favorable to the development of downy mildew and are carried out in the coolest hours. L'organic farming limits L'use of copper to 4 kg per hectare per year, since the metal it does not degrade and accumulates in the soil in the form of insoluble compounds with negative effects on microfauna and microflora.
There botrytis onion (Scaly Botrytis, Botrytis allii) belongs to the Sclerotiniacee family.B. squamosaattacks i young leaf tissues of the onion generating infections on the entire surface of the leaves, whileB. alliipenetrates the collar through the injured tissues infecting the bulb. The affected leaves have small light spots and die early. For their part, the attached bulbs become brown, soft and spongy. A'white-gray micellar outgrowth it makes its way between the scales of the bulbs and small black sclerotia develop on the external scales.
To prevent botrytis attacks, the onion must be grown in a different area from that in which it grew the previous year, carrying out rotations. In addition, it must be planted avoiding damaging the collar and positioning it at a density such as to favor the movement of air between the plants and therefore the drying of the leaves. To avoid leaf wetness, it is preferable to limit irrigation interventions and carry them out in the morning. If you irrigate at sunset, try not to wet the plants but only the soil.
Botrytis spreads rapidly in moderate temperatures (10-24 ° C) and high humidity. When the climatic conditions are suitable for the development of the fungus, it is advisable to start the interventions at the appearance of the first symptoms on the onion and repeat them every 7-10 days (every 15 days, if you use cupric products in the absence of rain). Among the substances that can be used for the defense against botrytis, the sodium bicarbonate, mixed with water and distributed with a backpack pump.
Also the Fusarioum (Fusarium oxysporum f.sp. cepae), a fungus of the Nectriacee family, can attack the onion, especially when temperatures reach 25-32 ° C. Ascomycete penetrates the vascular tissues weakening the plant starting from the basal leaves and determining browning of the stem, chlorosis, growth retardation, wilting is desiccation of the crop.
For to prevent the Fusarium attack, it is possible to intervene in different ways. On the one hand, crop changes prevent the onion from being grown on the same soil after a few years, since the fungus can survive for a long time in already contaminated soil and therefore attack the species even after a long time. On the other hand, they play a fundamental role in the prevention of fusarium rotations with grasses and legumes, the contribution of organic matter, the use of tolerant varieties and healthy propagation material, localized irrigation and soil drainage.