Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu
Blossom end rot has appeared in tomatoes throughout Maryland in the last few weeks (Fig. 1). This is mostly due to the very dry conditions we have had this month. Blossom end rot is caused by too little calcium reaching the cells on the blossom end of the fruit as it is developing. Calcium is dissolved in soil water and is taken up by the plant through the vascular system. During periods of high moisture stress, water containing calcium moves rapidly through the plant to the leaves with most of the calcium ending up in the leaves after transpiration has occurred. Since the fruit transpires very little, less calcium is deposited there; resulting in a localized calcium deficiency in the fruit. Almost all of the calcium that a mature fruit needs is within the fruit when it is the size of a nickel.
Figure 1. Several different ‘forms’ of blossom end rot on tomatoes
Therefore, the blossom end rot that appears on fruit that is about ready to be harvested had the critical period of inadequate calcium occur weeks earlier. Even a temporary water stress during early fruit enlargement can cause blossom end rot. Another cause of blossom end rot is over-fertilization, especially with nitrogen. The excess nitrogen stimulates vegetative growth, which increases the transpiration rate and further inhibits calcium accumulation in the fruit. Cultivars that grow quickly and produce large amounts of foliage tend to be more susceptible to blossom end rot. Therefore, reducing nitrogen levels will help reduce blossom end rot. The prevalence of blossom end rot also may be exacerbated when there is a low ratio of calcium to certain other nutrients such as potassium and nitrogen. About the only place where I have seen little blossom end rot on tomatoes is when they are in high tunnels where moisture levels can be more precisely controlled.