Jerry Brust, IPM Vegetable Specialist, University of Maryland; email@example.com
I know it may seem odd to see an article about sunscald or sunburn on leaves with the week of rain we just had, but leaves came in over the last week as the rain started and the damage had been done days before this. It is also possible that there will be a greater chance for sunscald in the coming days as growers try to get their transplants out. An area on the leaf turning papery white or tan is usually the first indication of scald on plants (Fig. 1).
Figure 1. Scald on crucifer leaf (top) and bean leaf (bottom)
Many of these plants were set in the field after coming straight out of the greenhouse or off the trailer. Before the rains we had a few days of very hot temperatures and intense sunlight. In figure one you can see that only certain parts of the leaf are scalded (these are the areas that had direct sunlight on them for an extended period) and the tissue next to the scalded area is still bright green. In the transplant production house plants are exposed to filtered light so the leaves are good at absorbing as much light as possible. The problem with taking plants straight from this type of environment to the field is that the plants at times are not ready for the extra UV light they are going to receive. The leaf tissue rapidly becomes desiccated with the extra light/heat exposure, causing light tan to white discoloration on the leaves and stems of sensitive plants. At times even established plantings can experience this as well, especially during an unexpected heat wave, which we had (believe it or not) as a heat wave is defined as 3 or more consecutive days of temperatures at or over 90°F. 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 with all the cool wet weather we had growers were forced to get their plants out when they could. Make sure to appropriately water and feed plants that have sunscald while they are recovering.