Phytophthora blight is caused by an organism called a “water mold.” Water molds look very similar to fungi, but they are actually more closely related to brown algae. The particular water mold that causes Phytophthora blight is called Phytophthora capsici (sometimes abbreviated as P. capsici). P. capsici is related to another water mold – Phytophthora infestans – which causes late blight on potatoes and tomatoes and destroyed potato crops during the Irish Potato Famine.
All cucurbits (squash, pumpkins, melons, and cucumbers) are very susceptible to Phytophthora blight, as are sweet and hot peppers. In addition, Phytophthora blight affects tomatoes, eggplants, snap beans, and lima beans, and can survive on some weed species (see list of susceptible crops and weeds). Infected fruits turn soft and “melt,” while whole plants wilt and die. Check out the Photo Gallery to see what this disease looks like on each of these hosts.
P. capsici reproduces most rapidly in warm and wet or humid weather by producing millions of short-lived, lemon-shaped spores on the surface of infected plants (see some pictures here!). These spores can be splashed from the soil to plants, or between plants, and they can also be carried by moving water in a field. Each one can also release 20-40 motile spores that can swim short distances through standing water or saturated soil towards plant roots. Both of these spores can be produced very rapidly, and only require the presence of a single isolate of P. capsici.
A second type of spore with much thicker walls is produced inside infected plant tissue, and requires the presence of at least two isolates of P. capsici. All P. capsici isolates can be classified as either A1 or A2 isolates, and these thick-walled spores are only produced when A1 and A2 isolates grow close to each other. Although these spores are produced more slowly, they are very important to the life cycle of Phytophthora blight because they can survive for years in the soil until a susceptible crop is planted. For this reason, once these thick-walled spores are in the soil Phytophthora blight is there to stay. More images of spores of the Phytophthora blight pathogen can be found here.
Phytophthora blight spreads mainly by 1) water and 2) human activities. As the name “water mold” suggests, Phytophthora blight thrives in wet conditions, and its spores can easily be washed off of infected plants and down rows in a field, following the drainage pattern. They can also be carried via run-off from an infested field into surface irrigation sources like ponds, streams, and rivers. Within a field, spores are splashed from soil or infected plants to nearby plants, or they are distributed throughout the field as humans, animals, and farm equipment move through or between fields. The picture to the right shows plastic booties worn at Cornell's Phytophthora Blight Farm to avoid moving P. capsici to other fields.
Because thick-walled overwintering spores of P. capsici remain in the soil after plant material has decayed, any movement of soil (on tires of farm equipment, footwear, or animal hooves) can move P. capsici from one field to another. Infected fruit and plant material may contain either the short-lived spores or the overwintering spores of P. capsici, and these spores can be transported long distances if fruits are harvested from one field, develop symptoms in transit, and then are dumped in another field.