Summer disorders include ozone damage, leaf roll (The Vegetable Grower)
(Editor’s note: Gordon Johnson a vegetable and fruit specialist with the University of Delaware.)
We are starting to see evidence of air pollution damage from ozone in sensitive vegetable plants.
Those vegetables most susceptible include potatoes, watermelons, cantaloupes, snap beans, pumpkins, and squash.
Damage is most common during hot, humid, hazy weather with little wind.
Air inversions — when warm air at the surface is trapped by even hotter air in the atmosphere above — lead to build up of air pollutants that cannot disperse and, consequently, plant injury.
The most common form of air pollution injury to plants is ozone damage.
Ozone is a strong oxidant and is formed by the action of sunlight on products of fuel combustion. I
t is moved from areas of high concentration (cities, heavy traffic areas) to nearby fields.
In 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.
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.
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.
Late spring and early summer is the time of the year that we often see leaf cupping and rolling disorders appear in vegetable crops that are not related to pests or chemicals.
This can be seen in tomatoes, peppers, potatoes, watermelons, beans, and other crops.
This is a physiological disorder that may have many contributing factors.
In tomatoes, leaf roll starts at the margins which turn up, then roll inward, most commonly on the lower leaves.
Upward cupping is also found commonly in watermelons and potatoes. Beans, peppers, and other vegetables may cup downwards.
Leaves may stay in this rolled or cupped state for a short period of time and then return to normal, or they may remain permanently rolled or cupped. Rolled leaves may become thicker but are otherwise normal.
There are several possible causal factors for physiological leaf roll or cupping.
Water relations are suspected in many cases where there has been a reduction in water uptake or increased water demand placed on the plant.
The plant responds by rolling the leaves which reduces the surface area exposed to high radiation.
High temperatures, excessive pruning, cultivation, and vine moving activities may also trigger leaf rolling.
High nitrogen fertility programs followed by moisture stress may also trigger this type of leaf roll.
In most cases, yields are not affected by physiological leaf rolling or cupping.
However, growers may choose to select varieties that are less susceptible to this disorder.
Growth regulator herbicides are often of most concern for drift damage to vegetable crops in spring and summer.
This group includes dicamba, 2,4-D, MCPA, MCPP, triclopyr, picloram, clopyralid, aminopyralid, and quinclorac.
These herbicides can drift over one mile from where they were applied when volatilized.
They are used in field crops, turf, and right of way weed control.
Symptoms include leaves becoming cupped, crinkled, puckered, strap-shaped, stunted, and malformed.
Leaf veins can appear parallel rather than netted, and stems become bent, twisted, and brittle.
This type of herbicide damage can lead to severe yield losses in vegetable and fruit crops.
When compared to physiological leaf roll, symptoms will be concentrated on the upper part of the plant (growing points), the leaf veins will be affected showing a parallel pattern, and stems may be twisted — none of these symptoms will be present in physiological leaf roll.
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