Potato Grower

clouds. • Carbon dioxide levels are dramatically increased by wildfires, and increased carbon dioxide usually leads to an increased rate of photosynthesis because normal atmospheric levels tend to be limiting. Ambient levels of carbon dioxide are estimated to be about 400 parts per million (ppm), but many plants will not reach their maximum uptake until closer to 1,000 ppm. • Air temperatures are often moderated by smoke. Since the optimal temperature for potato photosynthesis peaks around 90 degrees Fahrenheit, the smoke cover may be a positive factor for plant productivity if it helps to curb temperatures when they would otherwise be very hot. According to this assessment, light is the most limiting factor for photosynthesis when there is smoke. But the degree to which smoke limits photosynthesis and potato yields will depend on how dark it gets and how long the smoke persists. This is an oversimplification of the influences smoke can have on photosynthesis, since we should also consider that the chemical constituents of smoke can damage plant functions at a cellular level. And we need to recognize that plants can modify leaf structure (orientation, chlorophyll content, etc.) to maximize light captured under low light conditions when they are given some time to adjust. Smoke Changes the Weather Smoke can change the weather by blocking or scattering light and limiting radiant warming from the sun, much like clouds. These changes will affect the crop, so it's important to modify management accordingly. For instance, you might find that the crop needs less irrigation when it's smoky, because evaporative water loss is reduced when it's cooler and sunlight is less intense. Smoke, Air Inversions & Herbicide Drift Smoke tends to prolong thermal air inversion conditions because it inhibits some of the radiant warming of the ground layer that is needed to break up the warm inversion layer. Air inversions are often associated with herbicide drift, especially for lightweight herbicides like 2,4-D. So be careful when spot-spraying weeds when there is a smoky haze, because the herbicide may travel farther than you expect. Smoke, Prolonged Dew & Potato Diseases You may notice more dew on your crop in the mornings when it's smoky, and it may take longer to dry. The particles in smoke promote dew formation; they serve as condensation nuclei or tiny particles in the air on which water vapor can condense. The smoke can also help keep the dew around longer by blocking the sun's warming rays and slowing evaporation. The hot and dry summer environment in most potato-growing regions of the western U.S. discourages diseases like gray mold and late blight. But smoke cover can modify the environment so that it favors these diseases, and outbreaks have been known to occur. When it's smoky, keep a closer eye out for diseases that favor cooler, moist conditions, and be prepared to manage them more aggressively than you usually do. Smoke & Human Health Concerns If you want to know how wildfires (and other things) are affecting air quality, visit the EPA's AirNow website (www.airnow.gov) or your state's smoke information website (e.g., Idaho Smoke Blog, Washington Smoke Blog). These websites have maps that show current readings from sensors that monitor air quality. These websites also provide information about how to protect yourself when air quality conditions are poor. Carrie H. Wohleb is an associate professor and specialist of potato, vegetable and seed crops in Washington State University's Agriculture & Natural Resources Program Unit. She can be contacted at cwohleb@wsu.edu PG In this picture, the stippling is only where the leaf was exposed to air ; the area that has no symptoms was covered by other leaves. Generally, only the most exposed and photo- synthetically active leaves, usually in the upper canopy, show symptoms of ozone injury. Potatoes typically show a leaf stippling symptom when exposed to high con- centrations of ozone from wildfires. The stipples are usually found on both sides of the leaf. WWW.POTATOGROWER.COM 29

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