Thus, diseases where leaves are wilted because of root rot or clogging of the vascular system will not be discussed. In diagnosing diseases, it is important to examine all parts of the plant, including ones far removed from the obvious symptom. There are many foliar diseases caused by microorganisms, but this article will only treat the major ones in south Texas. Cercospora leaf spot, caused by the fungus, Cercospora citrullina , is the most prevalent foliar disease of watermelon in south Texas, but it is not usually a serious problem. Sometimes the area surrounding the brown spot is slightly chlorotic.
Heavy infection leads to defoliation and therefore chemical control is advisable.
I have seen the disease on new watermelon land and have noticed only healthy-looking citron melons and burgherkins in the midst of symptomatic watermelon plants. Airborne movement of spores is important for introducing the pathogen into fields. Downy mildew, caused by the fungus Pseudoperonospora cubensis , does not reliably appear in fields, but its occurrence should always be a cause for concern because of its ability to quickly increase and become uncontrollable. It is first seen as irregular, yellow areas on the upper leaf surface. The centers of the areas become brown and they are surrounded by a yellow area.