Gordon Johnson, Extension Vegetable & Fruit Specialist;gcjohn@udel.edu
There are a number of leaf disorders that can be seen at this time in vegetable crops not related to diseases or deficiencies.
Leaf Aging
Discoloration, bronzing, whitening, brittleness, and leaf tearing are common on older leaves of many vegetables, particularly cucurbits. In cucurbits, the oldest leaves (crown leaves) in watermelons, cantaloupes, squash, cucumbers, and pumpkins will have large areas that are discolored (white, tan, or bronze). These leaves will be brittle to the touch and may start to tear or shred with high winds and storms. This condition is common in cucurbit crops but also can be found in other vegetables such as tomatoes and can be due to a number of leaf aging factors including mineral nutrient scavenging (export of mobile nutrients from oldest leaves to newer leaves), ozone air pollution damage, chemical phytotoxicity (copper fungicides for example), repeated stress cycles, and wind injury. Leaf cells that die will leak their contents, releasing enzymes and oxidizing chemicals affecting nearby cells thus accelerating the “aging” process. This results in large patches of dead leaf cells that then dry, making the leaf feel brittle. If leaf veins are damaged, water and food transport will be compromised, accelerating leaf decline. This leaf aging is not to be confused with damage from mite feeding which is also concentrated on oldest leaves.
Ozone Damage
With the hot, humid weather, we are starting to see evidence of air pollution damage in sensitive vegetable plants. Vegetables most susceptible to air pollution 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.
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. 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. In muskmelons and other melons, the upper surface of leaves goes directly from yellow to a bleached white appearance. Ozone injury on squash 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.
Physiological Leaf Curl
Leaf cupping and rolling in vegetables can be caused by virus diseases, aphid infestations, herbicides and growth regulators. However, 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. Physiological leaf roll or cupping is often variety dependent with some varieties being more susceptible than others.
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. Inadequate calcium moving to leaf margins may also cause a different type of leaf cupping. This is also related to interrupted water movement.
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.