Results of Wet Condition on Watermelon and Cantaloupe

The 2019 production season started with above-normal rains. The wet conditions affected agriculture production, including watermelon and cantaloupe. In this article, we will review some of the watermelon and cantaloupe problems that are often associated with wet conditions.

Manganese toxicity– This nutrient disorder occurs more often on cantaloupe that is grown in soils with pH lower than 5.5. Although liming before planting is a common practice, it is not unusual that we see soil pH that has dropped below 5.5 in sandy soil, especially during wet years. Manganese exists in soil solution as either reduced (Mn2+) or oxidized (Mn3+) form. Plants take up manganese in the reduced form (Mn2+). The proportion of exchangeable Mn2+ increases dramatically as soil pH decreases, and this reaction is promoted in waterlogged soils with low oxygen condition. As raindrops fall through the air, they dissolve CO2 and form enough carbonic acid to lower the pH of the water from 7 to about 5.6. The acid in precipitation contributes to soil acidification, another reason manganese toxicity occurs in wet years.

If manganese toxicity is detected during the season, there is not much we can do to alleviate the problem, especially when fertigation is not applicable. If the system is set up to apply fertigation through drip tapes, using fertilizers with a nitrogen source from nitrate-nitrogen (calcium nitrate and potassium nitrate) instead of ammonium-nitrogen may help by increasing soil pH. Potassium carbonate can also raise soil pH. It is water soluble and can be applied through drip systems. However, as correcting soil pH can be a prolonged process, it may be too late to see a yield response in the current season if the symptom has already been noticed.

Manganese toxicity can mimic a disease. It may seem to spread, but actually the plants in the lowest pH soil show the symptoms first. Then the symptom ‘spreads’ to other areas with soil pH a bit higher. Alternaria leaf blight is the disease most commonly mixed up with Manganese toxicity. It is important for growers to learn the symptom and address the problem in right direction. Please refer to this article Manganese Toxicity on Cantaloupes for detailed symptom description and addition information about this problem.

Watermelon hollow heart- is mainly caused by poor pollination. When the pollenizer plants (diploid watermelons) are located further away from the seedless plants, they are more likely to develop hollowheart. In addition, cold weather and the lack of bee movement during fruit set also causes poor pollination and increases the chance of hollowheart.

The critical time for fruit set is typically happened around 5-6 weeks after transplanting. Continuous cloudy and rainy days during fruit set period will affect bee movement, and is likely to increase the chance of hollow heart. With the harvest still a few weeks ahead, hollow heart has not been reported yet this year. However, early planted fields may have experienced more than usual rainy and cloudy days in the early half of June this year.

It is important to note that there is a significant varietal difference for hollow heart. A list of hollow heart ratings among 38 seedless watermelon varieties can be found in 2018 Watermelon Variety Evaluation in Indiana on page 14. More information about watermelon hollow heart can be found in the article Hollowheart of Watermelons.

Mature watermelon vine decline– commonly known as watermelon sudden wilt. It happens late in the season, a few days before first harvest or right after. The resulting vine wilt and collapse may cause poor-quality fruit. Although the definite cause of mature watermelon vine decline has not been determined, it is generally agreed that the symptom is associated with poor root system that cannot adequately supply the plant with water and nutrient late in the season. Watermelon develops a much more robust root system under drier conditions when the roots are forced to explore water deep in the soil. With constantly wet soil, roots lose the capability to grow deep. In addition, it places oxygen stress that results in additional physiological stress and perhaps necrosis of fine roots. As a result, mature watermelon vine decline often appear after heavy rains, and appear first in low, poorly drained areas. More information about mature watermelon vine decline can be found in this article Late-season Vine Declines of Melons: Pathological, Cultural and Both?

Phytophthora blight– This disease is more likely to develop during periods of heavy rains in relatively poorly drained soils. Recently, we observed this disease on both watermelon and cantaloupe. Large, soft lesions develop on fruit, typically close to where it comes into contact with the soil, and make fruit ripen prematurely. Yield loss caused by Phytophthora blight can be dramatical. Considering the excessive rains this season, Dr. Dan Egel is recommending growers apply specialized fungicides for Phytophthora blight. More information about this disease can be found in this article Phytophthora Fruit Rot of Watermelon. Recommended fungicides and spray schedule can be found in Midwest Vegetable Production Guide and purdue.ag/melonfs.

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