Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu
I know it may seem odd to see an article about sunburn on leaves with the week of overcast rainy skies we have had, but pictures of damage to crops came in the last few days after some bright but brief sunshine. Plant leaves turning a papery white or tan can indicate sunburn or sunscald on plants (Figs. 1 and 2). These symptoms on vegetable plants also can be caused at times by cold or wind damage as discussed by Gordon Johnson in a previous article. Sunscald on plants can occur when plants are set in the field after coming straight out of the greenhouse or off the trailer without being thoroughly hardened off. Only certain parts of the leaf will be scalded (these are the areas that had direct sunlight on them for an extended period of time) and the tissue next to the scalded area is still bright green (Fig. 1). In the transplant production house plants are exposed to filtered light so they grow leaves that are good at absorbing as much light as possible. The problem with taking a plant straight from this type of environment to the field is that the plants at times are not ready for the extra UV rays they are going to receive. The epidermal layers of leaf tissue desiccate (burn) with the intense sun exposure, causing light tan to white discoloration on the leaves and stems of tender plants. At times even established plantings can experience this as can be seen with the Swiss chard in figure 2. Once leaves are damaged, all that can be done is to support the plant until it manages to grow new leaves. Hardening off the transplants would have prevented the sunscald on the new transplants, but I rarely do this myself and have not had much of a problem for not doing so. Make sure to appropriately water and feed plants that have sunscald while they are recovering and watch for any secondary foliar infections in the damaged tissue.
Figure 1a. Sunscald on a crucifer leaf
Figure 1b. Sunscald on a bean leaf
Figure 2. Sunscald leaf of Swiss chard