Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
Watermelon maturity and harvest was accelerated this year by the high temperatures in June and July and there may be a shortage of late watermelons. With this in mind, many growers are trying to preserve later sets in early harvested fields to mature at the end of August or in early September. Success will depend on a number of factors.
First and foremost is the health and condition of the vines. Plants need to have an adequate amount of healthy leaf area to support watermelon fruit development. The leaves closest to the developing fruit are the most critical. If these leaves are in good shape, then an aggressive fungicide program is warranted to keep them healthy and provide photosynthates for the late set.
Previous harvest operations will have spread diseases in the field and caused wounds that further accelerated disease development. Older portions of watermelon vines with fungal infections will serve as reservoirs to infect healthy leaves making protection more difficult. Good coverage will be essential. Mix fungicides with some local systemic activity with a protectant fungicide. This time of year you may see the full mix of diseases including gummy stem blight, Alternaria, and anthracnose along with the potential for some late season diseases such as downy mildew and/or powdery mildew. The fungicide program that you choose needs to target diseases that are occurring in your field and in neighboring fields, so intensive scouting and correct diagnosis of existing diseases is critical.
In addition, mite, aphid, rindworm, and other insect populations may have built up to high numbers and aggressive control programs will also be needed to protect late sets.
It is also critical to evaluate the condition of the stems. Vines are damaged by trampling during harvest. When a vine is stepped on, it compromises the ability of that stem to carry water and mineral nutrients to leaves and fruits further down the vine. Areas with heavy foot traffic will have a lower percentage of saleable fruits from late sets because of this.
Another critical factor is the presence of any mature watermelons still attached to vines that were not harvested in prior trips across the field (melons were missed, had defects, too small, had damage, etc.). When a mature fruit is kept attached to a vine, it will delay the development of any later sets (due to hormone signals in the plant). Detaching or removing these melons will help later sets progress. A plant will mature 2-3 watermelons in the normal harvest window. To get a plant to produce more, all old fruits must be removed promptly.
Water is one of the most important factors in maturing late sets. Often, less attention is paid to irrigation during and after harvest operations. Beds are allowed to dry and vines are water stressed. This also shifts the hormone balance in the plant and can impede later sets or cause premature abortion of late sets. It is important to keep fields with later sets well watered and manage to irrigation closely.
Finally, additional fertilizer will be needed to promote the later sets, maintain healthy leaves, and promote fruit development. Nitrogen and potassium are the most critical in this regard. An additional 20-50 lbs of N/acre should be applied through the irrigation. 20-50 additional lbs of potassium per acre can also be of benefit, especially in light, low CEC soils (potassium is important for fruit quality).