Greenhouse onion cultivation is widely valued for its efficiency and productivity gains. However, these facilities can become a breeding ground for diseases like Botrytis due to their characteristic conditions of high humidity , limited ventilation, and moderate temperatures. The main associated risk is the development of various Botrytis species, which can seriously compromise both crop yield and quality.
What is Botrytis in onion?
Botrytis encompasses several fungal species, primarily Botrytis squamosa and Botrytis cinerea , although Botrytis alli and Botrytis aclada can also be found . These pathogens can infect onion leaves, stems, necks, and bulbs , causing anything from mild leaf spots to severe neck and bulb rots, which are responsible for significant economic losses and a reduction in the commercial quality of the product.
Fungi of this genus survive in infected plant debris , bulbs, and soil, as well as in storage facilities. When favorable conditions are present, such as moderate temperatures and high humidity , they produce spores that are easily dispersed by wind, water, or contaminated tools, generating new infections both in the field and in storage.
Botrytis in onions can develop at any stage , being especially damaging during active growth and the ripening and storage periods. Furthermore, its potential to affect other horticultural and ornamental crops makes Botrytis an agricultural problem of global significance.
Conditions that favor Botrytis infection
- High and prolonged relative humidity: Typical condition in greenhouses, rainy climates or with excessive irrigation.
- Optimal temperatures between 15°C and 25°C: Even though B. squamosa initiates infections even from 10°C.
- Remains of dead tissue, cuts, wounds or previously infected bulbs.
- Limited air circulation: Promotes the maintenance of surface humidity in leaves and stems.
- Condensation Frequent on leaves: It can be the result of dew, rain or poorly managed sprinkler irrigation.
- High planting densities and crops with low light, increase the incidence.
Life cycle and dissemination of the Botrytis fungus
Understanding the development of Botrytis is essential for effective control. The fungus can remain dormant (as sclerotia or mycelium) in crop debris or affected bulbs for extended periods. The initiation and perpetuation of the cycle include:
- Spore formation in infected tissues (especially with high humidity).
- Dispersions by wind, water, tools or contact between neighboring plants.
- Adhesion and germination on leaf surfaces, bulbs or neck areas.
- Penetration through wounds, natural openings or enzymatic action.
- Colonization and development of grayish sporulation (dust/mold visible in humid conditions).
- Latent persistence in case of adverse conditions until the environment becomes favorable again.
Botrytis symptoms in onions: how to identify them?
Early and accurate identification is key to successful control. Botrytis can be confused with other diseases, but it has its own distinct clinical characteristics:
- Small whitish or gray spots (1-5 mm), circular or elongated, on the upper leaf surface, often surrounded by a light green or silver halo with a watery appearance in early stages.
- Sinking and straw color in the center of advanced lesions, sign of necrosis.
- Longitudinal groove characteristic in mature lesions.
- Wilt and retrograde necrosis from tips and margins, resulting in progressive leaf death.
- Appearance of grayish or moldy dust (sporulation) on lesions when humidity is high.
- Damage to bulbs and neck: soft and watery rot, greyish-brown spots, reduction in size and quality.
- Yellow patching of dying plants in the field and premature leaf fall.
Differentiation from Alternaria: Unlike Alternaria porri , which produces large lesions with dark concentric rings (target type), Botrytis shows smaller, paler, scattered spots without these rings.
Botrytis species in onions and similar diseases
In onion cultivation, several species of Botrytis and other similar pathogens can interact. Botrytis squamosa is the main cause of leaf spot and gray mold; Botrytis cinerea causes gray mold; and Botrytis alli is associated with crown rot.
Alternaria porri causes leaf blight, presenting similar but distinguishable symptoms. Correct identification allows for the selection of the most appropriate treatment, as some fungicides or practices may be more effective against one pathogen than another.
How to combat Botrytis in onions: integrated strategies
An integrated and preventative approach is the best tool. It combines cultural, biological, chemical, and monitoring measures to protect your crop.
Elimination and destruction of sources of infection
- Remove and destroy (burn or remove from the field) infected leaves, plants and bulbs.
- Promptly remove piles of onion tops or debris after harvest to prevent them from persisting in the environment.
Application of fungicides (chemical management)
- Use authorized fungicides and specific for Botrytis (such as iprodione, pyrimethanil, fenhexamide, fluazinam, cyprodinil+fludioxonil, chlorothalonil, mancozeb, boscalid, azoxystrobin, tebuconazole, pyraclostrobin, copper oxychloride).
- Alternates active ingredients for prevent resistance in the mushroom.
- Foliar spraying is the most common method, although the soil fumigation It may be useful in initial phases or in localized foci.
Biological and natural management
- Biofungicides based on Bacillus subtilis (especially the QST 713 strain) offer preventive protection, being a less aggressive alternative to conventional chemical products.
- The use of Trichoderma harzianum It is booming and shows promise in integrated management programs, although the evidence in onions is still incipient.
- Promotes rapid wound healing to prevent the entry of fungus.
Cultural and agronomic management practices
- Sowing at adequate density: Ensures good ventilation and less moisture build-up between plants.
- Crop rotation: Alternate onions with non-host species for at least two crop cycles.
- Varietal selection: Choose resistant or short-cycle onion varieties to minimize risk in endemic areas.
- Keep the field free of weeds, volunteer onions and residues from previous harvests.
- Avoid sprinkler irrigation and prioritizes localized irrigation to keep foliage dry.
- Ensures excellent drainage and do not overwater.
- Limit the use of late fertilizers and promotes uniform ripening of the bulb.
Preventive measures to avoid Botrytis in onions
- Certified and healthy seeds: Avoid spreading the fungus from planting.
- Do not move bulbs from infected areas to other fields or warehouses.
- Respect the safety distance between fields intended for seed production and those for commercial production.
- Monitor the crop frequently: Allows for early detection and rapid action.
- Periodic disinfection of tools and machinery agricultural to prevent indirect transmission.
- Improve ventilation and, if possible, use heating in a greenhouse to keep the ambient humidity low.
- Soil solarization: Useful in warm climates to reduce fungal inoculum before planting.
- Use of plastic covers with UV filter: Reduces fungal sporulation in covered crops.
Storage and post-harvest care
- Cure the bulbs freshly harvested at high temperatures (32-50°C) for 2-4 days to reduce humidity, then stored in a cool, dry environment (around 3°C).
- Eliminates bulbs with symptoms before storage, as they can be a source of infection in closed spaces.
- Avoid storing wet bulbs or with visible lesions to prevent the disease from progressing.
Common mistakes in managing Botrytis in onions
- Do not remove or destroy plant debris or volunteer onions after the harvest.
- Excessive irrigation, especially by sprinkling and lack of adequate drainage.
- Do not alternate fungicides, favoring the resistance of the pathogen.
- Delay monitoring and removal of initial outbreaks, allowing rapid spread.
- Storing bulbs in humid conditions or with physical damage.
Frequently asked questions about Botrytis in onions
- Can Botrytis coexist with Alternaria?
Yes, it is common and differentiation is essential for successful treatment. - Is chemical control always necessary?
No, integrated and preventative management is preferable. Fungicides are a last resort and should be used alternately to avoid resistance. - Does Botrytis affect bulbs during storage?
Yes, that's why proper drying and careful selection before storage are essential. - Why is humidity so important?
La high humidity and poor ventilation They are the factors that most favor the infection and spread of the fungus.
Successful control of Botrytis in onions depends on meticulous crop monitoring, coordinated application of preventative and corrective strategies, and accurate identification of the pathogen and its symptoms. Focusing on prevention, integrated pest management, and rapid intervention ensures healthier, more productive crops and a higher-quality onion harvest, reducing reliance on chemical products and improving agricultural profitability.
