Mildew management in melon cultivation

The melon (Cucumis melo L.) is one of the most consumed and cultivated vegetables in the country, as in addition to its nutritional properties, its cultivation has been very profitable. Among the problems faced by producers, diseases have been a cause for concern as they directly affect productivity and fruit quality. The disease known as downy mildew is very common and can become important, especially in regions where mild temperatures in the range of 15ºC to 27ºC predominate.^oC and high relative humidity, above 90%. When uncontrolled and under favorable conditions, it causes early defoliation and harms melon productivity, and can cause 100% losses when the attack is severe. Initially, small irregular to rounded yellowish spots appear on the upper surface of the leaves. On the lower surface, there is the presence of waterlogged tissue where the fungus fructifications (sporangia and sporangiophores) of light color are subsequently formed, which can be confused with another disease called powdery mildew. As symptoms develop, the yellowish spots become brownish necrotic, forming angles with the veins.

Symptoms of downy mildew on melon leaves showing small initial yellowish lesions and other more developed lesions with a necrotic brown center.

Photo: G.R. Santos

Etiology and epidemiology

Downy mildew is caused by the fungus Pseudoperonospora cubensis (Berk. et Curtis) Rostowzew. The fungus belonging to the Oomycetes class, Peronosporaceae family, is considered an obligate parasite. Under microscope observations, the pathogen presents mycelium without a septum (cenocytic) that forms mainly on the lower surface of the leaves. The sporangiophore has dichotomous branching in the upper third and ovoid to ellipsoid sporangia are formed at its ends (Figure 1).

Figure 1 - Life cycle of Pseudoperonospora cubensis, causal agent of melon downy mildew.

Source: G.R. Santos

It is unlikely that P. cubensis survive from one year to the next in cultural remains, in the field. The fungus can produce oospores (sexual spores) on old tissue, but they are rarely found. These oospores can remain on infected crop debris. Out-of-season plantings and other cultivated plants from the cucurbitaceae family, such as watermelon, cucumber, pumpkin and also wild/native plants, such as, for example, the São Caetano melon, can guarantee the survival of the fungus of a year to year, in the field.

The spread of the fungus, under favorable conditions, occurs through wind, rain splashes or sprinkler irrigation. Sporangia formed into sporangiophores are easily carried great distances by wind.

High relative humidity, close to 100%, and temperature in the range of 15^oC to 27^oC favor the formation of free water and the infectious process occurs from the stomata of the leaves and consequently the development of the disease. In the Formoso Project, located in the state of Tocantins, melon planting is done in the off-season, which varies from May to August and despite the irrigation system adopted does not wet the leaves (sub-irrigation) and the relative humidity at this time during the day is close at 40%, the disease appears in the month of July due to the temperature at night falling below 35^oC to 25^oC, thus creating a thermal gradient. In this way, leaf wetness occurs, which begins around 21 pm and continues until 8 am the following day (enough time for the pathogen to develop in plant tissues under these conditions).

Integrated Management

It involves the use of a set of control measures that must be adopted throughout the production process. For melon cultivation, the following measures are recommended:

1 - Do not use sprinkler irrigation, however, if necessary, avoid irrigating at night and early in the morning, to reduce the period of leaf wetness;

2 - Do not grow melons in seasons subject to rain;

3 - Incorporate crop residues after harvesting the fruits, as the pathogen that causes downy mildew does not have the ability to survive in the soil for a long time;

4 - Avoid planting in poorly drained and humid areas that favor leaf wetness;

5 - Cultivate crops so that new planting areas are not downstream of older crops of other plants in the Cucurbitaceae family;

6 - When possible, plant resistant or tolerant varieties and/or hybrids;

7 - If necessary, lime 30 to 40 days in advance of planting and mineral fertilization according to the results and interpretation of the soil analysis;

8 - Avoid excess nitrogen fertilization;

9 - Avoid planting the same variety or hybrid in large areas and for a long time;

10 - Use healthy seeds and avoid too dense cultivation, as in this situation there is shading and there is no ventilation between the leaves, favoring leaf wetness and the development of the disease;

11 - Rotate crops with plants from other families, such as grasses or legumes, for three to four years;

12 - Roguing the planting periodically, seeking to eliminate severely attacked plants;

13 - Chemical control should preferably be carried out preventively with contact fungicides in rotation with systemic ones, respecting the manufacturer's dose, safety interval and using products duly registered for the crop, applied alternately and also in mixtures when recommended, to avoid the emergence of resistance;

14 - Use fungicides registered for melon, based on metalaxyl, chlorothalonil or copper products, and other molecules belonging to various chemical groups can also be applied, such as strobilurins, triazoles, dicarboxamides, benzimidazoles, etc.; always following due care and recommendations from an agronomist or other qualified professional.

Click here to read the article in Revista Cultivar Hortaliças e Frutas, issue 79.