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Predicting late blight in melon

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Javier Méndez Lorente
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Table of contents: Predicting late blight in melon

late blight, caused by the pathogen Pseudoperonospora cubensisis a devastating disease affecting cucurbits, including melons. This disease is characterised by its rapid spread and severe impact on crop yield and quality. It is crucial to identify and control downy mildew early to minimise damage and ensure a healthy crop.

Defoliation caused by late blight not only reduces plant vigour but also exposes the fruit to sunburn, affecting crop quality. The loss of foliage decreases the photosynthetic capacity of the plant, resulting in lower fruit size and quality, and also decreases the sugar levels of the melon.

Characteristics of the pathogen

Pseudoperonospora cubensis is an oomycete, a fungus-like organism belonging to the Kingdom Protista, specifically to the phylum Oomycota. This pathogen, responsible for downy mildew on melon and other cucurbits, is more similar to algae than to true fungi. It has characteristics that facilitate its infection and spread, such as the production of sporangia, reproductive structures containing motile, biflagellated zoospores, which initiate infection. In addition, it develops coenocytic hyphae, multinucleate filaments without transverse septa that extend into host plant tissue, absorbing nutrients and causing damage.

In its life cycle, P. cubensis forms sporangia on the surface of infected leaves under conditions of high humidity and moderate temperatures. These sporangia are dispersed by wind and water (dew or rain). When free water is present, the sporangia release motile zoospores that swim into the leaf stomata, where they germinate and initiate infection. The zoospores penetrate the leaf tissue, developing hyphae that spread between the cells and form haustoria, structures that extract nutrients from the plant cells. As the infection progresses, the pathogen continues to produce sporangia on the lower surface of the leaves, perpetuating its life cycle.

Identification of Late Blight in Melon

Initial Leaf Symptoms:

  • Yellowish stains: The first signs of late blight manifest themselves as irregular chlorotic (yellow) spots on the upper surface of the leaves. These spots are usually confined between the main veins of the leaf, giving it a characteristic pattern. These yellowish areas may appear oily or wet, especially in the early stages of infection.
  • Distribution of Stains: Initially scattered, the patches may cluster and form larger areas of discolouration as the disease progresses. As the patches enlarge, they may coalesce, creating extensive areas of affected tissue.

Evolution of the Stains:

  • Colour Change: Yellow spots evolve to darker colours, turning brown to black due to tissue necrosis. This darkening is an indication of death of the affected leaf tissue.
  • Growth and Merger: The spots tend to spread and coalesce, creating large necrotic areas that compromise the photosynthetic capacity of the leaves. This can lead to a significant loss of functional leaf area, impacting overall plant health.
  • Greyish Felt: Under conditions of high humidity, it is characteristic to observe a growth of sporangium of Pseudoperonospora cubensis on the underside of the leaves. This growth appears as a light grey to mauve felt and is especially visible in the early morning hours or in damp environments. This distinctive sign helps to confirm the presence of the pathogen.

Additional Signs:

  • General Appearance: Affected plants show a decrease in overall vigour, with leaves showing a wilted appearance and significant areas of necrosis. This loss of foliage affects the plant's ability to carry out photosynthesis, which can further weaken the plant.
  • Stem and fruit: In severe cases, infection can spread to stems and fruits, although this is less common. Affected fruits may show superficial lesions and decay if in contact with infected leaves. The presence of dead tissue on fruits may compromise their quality and marketability.

Favourable conditions for Late Blight

Late blight on melons and cucurbits thrives in high humidity environments. humidity y warm temperatures:

  • High HumidityMoisture on the leaves, caused by rain or dew, and high relative humidity in the environment, are crucial for germination and dispersal of the fungal spores.
  • Warm TemperaturesThe pathogen thrives best between 18 °C and 25 °C and is most active in warm and humid seasons, such as spring and summer.
  • DispersionWind and water facilitate the spread of the fungus, carrying spores to new areas and accelerating infection.

Control and Management Strategies

Regular MonitoringCheck crops frequently to identify early symptoms of the disease.

Crop RotationAlternate melons with non-cucurbit species for 1 to 2 years.

Stress ManagementProvides balanced nutrition to reduce stress on plants.

HygieneClean and disinfect tools and equipment to prevent the spread of disease.

Irrigation MethodsUse drip irrigation instead of sprinkler irrigation to minimise leaf wetness.

Avoid Wet PlantsDo not work on plants when they are wet to prevent the spread of spores.

Waste Management: Remove and destroy plant residues at the end of the season by burying or burning them at a distance.

Chemical Management:

Systemic fungicides: Use systemic fungicides in combination with contact fungicides for effective control. Examples of fungicides include metalaxyl, mefenoxam and azoxystrobin. Be sure to alternate products with different modes of action to prevent resistance. In addition, sulphur dust application may be useful as part of integrated disease management.

Regular Application: Make weekly applications especially when weather conditions are favourable for the disease.

WatchDog. New control strategies.

Through weather stations WatchDogWith these devices, it is possible to monitor the environmental conditions of your farm live. These devices provide access to a detailed history of climate data such as solar radiation, relative humidity, temperature, rainfall, evapotranspiration (ET) and vapour pressure deficit (VPD). This information is available in real time from any device, facilitating the management and constant monitoring of the agricultural environment.

The SpecConnect platform, which is used to access the data from WatchDogincludes disease prediction models, providing valuable tools for decision making. In the case of downy mildew in melon, the Hyre Model and the Wallin Model are used to assess the risk and need for treatment.

Hyre model

  • Initial Condition: Warnings are activated after 10 consecutive days of favourable rainfall conditions.
  • Fumigation ConditionFumigation is recommended if 5 of the last 7 days are favourable for rain.
  • Definition of Favourable Rainy Days:
    • They are considered favourable if there has been 3mm of rain in the last 10 days and the temperature remains below 25°C.

Wallin Model

  • Initial ConditionActivated when the cumulative severity value exceeds 18.
  • Gravity ValueBased on the number of hours with relative humidity above 90% and the average temperature.

These models provide cumulative disease severity levels and generate alerts on when spraying is required. In addition to managing downy mildew in melon, SpecConnect uses the best available predictive models for other diseases and pests, offering a comprehensive solution for crop protection.

This combination of advanced technology and predictive modelling allows farmers to optimise the health of their crops and maximise yields, ensuring a timely and accurate response to environmental threats.

As in the case of this weather station with humidity probes installed on a melon farm in Mazarrón, Murcia, Tiloom offers the best products, as well as an installation, commissioning and after-sales service that places us at the forefront of the sector.

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