Powdery mildew of cucurbits in Florida
Powdery mildew is a common and serious disease of cucurbit crops in Florida. This disease occurs in cucumbers, muskmelons, honeydew, squash, gourds, and pumpkins grown both in field and greenhouse conditions. Previously, powdery mildew was an occasional problem for watermelons, but for the past 5 years the incidence of powdery mildew outbreaks has increased (Roberts and Kucharek 2005). A powdery mildew infection acts as a sink for plant photosynthates causing reductions in plant growth, premature foliage loss, and consequently a reduction in yield. The yield loss is proportional to the severity of the disease and the length of time that plants have been infected (Mossler and Nesheim 2005). For instance, in cucumber there is a negative linear relationship between disease severity and yield (Dik and Albajes, 1999). If this disease is not controlled in a timely manner, symptoms can be severe enough to cause extensive premature defoliation of older leaves and wipe out the crop.
In muskmelon and honeydew (Cucumis melo L.) plants, advanced foliage infection frequently results in decreased fruit sugar content, therefore reducing final fruit quality and marketability. In other cucurbits, such as yellow squash and zucchini, direct damage to the fruit can reduce yield and quality. According to Mossler and Nesheim (2005), powdery mildew occurs to some extent every year and may not be economically damaging in all affected cultivated areas. Powdery mildew infection is present in roughly 70% of the squash acreage in Florida.
In cucurbits, powdery mildew is caused by three fungal species, Podosphaera xanthii (syn. Sphaerotheca fuliginea auct. p.p.), Golovinomyces cucurbitacearum (syn. Erysiphe cichoracearum auct. p.p.), and Golovinomyces orontii (syn. Erysiphe cichoracearum auct. p.p.) (Jahn et al. 2002). However, Podosphaera xanthii (syn. Sphaerotheca fuliginea auct. p.p.) is actually more widespread than the other two species (Figure 2). Within each fungal species that causes powdery mildew disease, different races have been described (Jahn et al. 2002). These powdery mildew races have the potential to attack several powdery-mildew-tolerant or -resistant cucurbit crops if specific environmental conditions are favorable for fungal infection and spread.