Will Smoke from Wildfires Affect Vegetable Crops?

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

I was recently asked if smoke from wildfires will affect vegetable crops. The answer is complex and largely dependent on how long the wildfires continue to burn and weather patterns bring the smoke into our region.

When Emmalea Ernest and I were in eastern Washington State several years ago they had gone through a period where smoke from fires in British Columbia covered the seed growing region of that state for over a month. Researchers commented that crops stopped growing and were delayed in maturing for at least 4 weeks.

This is because smoke limits sunlight reaching crops and thus reduces photosynthesis. In addition, temperatures are reduced, which also slows growth. Deposits of fine particles from smoke on plants can also reduce effective sunlight until washed off by overhead irrigation or heavy rain.

Another consequence of wildfire smoke can be an increase in ozone levels. Ozone is a major cause of air pollution and can damage plants. Certain vegetables are highly susceptible to ozone injury.

Those vegetables most susceptible include potatoes, watermelons, cantaloupes, snap beans, pumpkins, and squash.

Ozone injury in susceptible vegetable varieties develops when ozone levels are over 80 ppb for four or five consecutive hours, or 70 ppb for a day or two when vegetable foliage is at a susceptible stage of growth. Ozone levels for Delaware are at 64 ppb, near the critical level for damage. Ozone levels are predicted to be 77 on Saturday, June 10. (DNRC’s Air Quality Forecast can be found at: https://dnrec.alpha.delaware.gov/air/quality/forecast/)

On potatoes, symptoms of ozone damage occur on the most recently emerged leaves and can be seen as a black flecking. Early red varieties are most susceptible.

Injury on watermelon leaves consists of premature chlorosis (yellowing) on older leaves. Leaves subsequently develop brown or black spots with white patches. Watermelons are generally more susceptible than other cucurbits to ozone damage. 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. 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. “Ice box” types are the most susceptible.

watermelon ozone damage

Ozone injury on watermelon

In muskmelons and other melons, the upper surface of leaves goes directly from yellow to a bleached white appearance.

Ozone 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.

ozone injury on squash

Ozone injury on squash. Note leaf yellowing.

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).

Ozone injury can be easily misdiagnosed as mite injury, pesticide phytotoxicity, or deficiencies.

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.