Gordon Johnson, Extension Ag Agent, Kent Co.; gcjohn@udel.edu
With the hazy, hot, humid weather of late June and early July we are seeing signs of air pollution damage to susceptible crops including potatoes, watermelons, cantaloupes, beans, pumpkins, and squash. Air pollution injury can be easily misdiagnosed as mite injury, pesticide phytotoxicity, or deficiencies. Atmospheric oxidants including ozone, nitrogen oxides, and peroxyacyl nitrates are the main causes of this injury, with ozone damage the most common. Ozone is formed by the action of sunlight on products of fuel combustion. It is moved from areas of high concentration (cities, heavy traffic areas) to nearby fields by wind.
Air pollution injury in susceptible vegetable varieties develops under the following conditions or situations:
● Ozone levels over 80 ppb for four or five consecutive hours, or 70 ppb for a day or two when vegetable foliage at a susceptible stage of growth.
● High levels of automobile exhausts. Crop injury is often visible on fields in close proximity to roads, especially with heavy summer weekend traffic.
● Humid conditions with cloudy, hazy overcast days and little breeze.
● High concentration of pollutants at ground level and in low lying areas. High pollution indexes in Baltimore and Washington are a good indication that this is occurring.
● Foggy conditions and heavy dews.
In potatoes, symptoms of ozone damage occur on the most recently emerged leaves and can be seen as a black flecking. Crop stage is also a factor with potatoes being most susceptible during the tuber bulking stage. Plant stress will predispose potato plants to ozone injury. Potato varieties vary in their susceptibility with Red Norland being very susceptible. Yukon Gold will also show ozone injury.
Ozone injury on potato
Injury on watermelon leaves consists of premature chlorosis (yellowing) on older leaves. Leaves subsequently develop brown or black spots with white patches. In muskmelons and other melons, the upper surface of leaves goes directly from yellow to a bleached white appearance. Watermelons are generally more susceptible than muskmelons to ozone damage and damage is more prevalent when fruits are maturing or when plants are under stress. Injury is seen on crown leaves first and then progresses outward. There are great differences in variety susceptibility with some older varieties such as Sugar Baby and Crimson Sweet watermelon and Superstar muskmelon being very susceptible. Seedless watermelon varieties tend to be more resistant to air pollution injury than seeded varieties, so injury often shows up on the pollenizer plants first. Ozone damage ratings were done on 60 diploid (seeded) and triploid (seedless) watermelon cultivars at North Carolina State University in 2000-2001. A large number of these varieties are still being used here on Delmarva. Pictures and ozone damage ratings from this research can be found at http://apsjournals.apsnet.org/doi/pdf/10.1094/PDIS.2003.87.4.428. Injury on squash and pumpkins is intermediate between watermelon and cantaloupe starting with yellowing of older interior or crown leaves. These leaves subsequently turn a bleached white color with veins often remaining green.
In snap and lima beans, ozone causes small bleached spots giving a bronze appearance on upper leaf surfaces and pods. Leaves may ultimately turn chlorotic and senesce (drop).
The key to avoiding air pollution injury is to plant varieties that are of low susceptibility and to limit plant stresses. Certain fungicides such as thiophanate methyl (Topsin and others) offer some protection against ozone damage. Antioxidants such as ascorbic acid and EDU (ethylene diurea) have been tested as protectants against ozone damage and some have shown promise.
Ozone injury on watermelon
Ozone injury on pumpkins