Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu
This past week we had thunderstorms come rolling through parts of the mid-Atlantic region. These storms often had damaging winds associated with them, but what might be worse is they dropped tremendous amounts of rain in a very short period of time. Some fields received 3-5 inches of rain. This level of rain can lead to several problems in the coming weeks in vegetables.
One problem is flooded fields and standing water, possibly for days. This is occurring and is predicted to occur more frequently not only in our area but throughout the Midwest. There are physiological effects on plants that occur in a field when there is standing water for any length of time, but there are other problems that could occur when growers use grafted plants to protect their crop from soil diseases.
Grafting vegetable crops has increased dramatically in the last 10 years to the point where most growers are producing at least one grafted vegetable crop. Most grafting is done to manage a soil disease problem such as Fusarium wilt, Fusarium crown and root rot, southern wilt, corky root rot or root knot nematodes. In the past when growers were faced with a soil disease problem they would fumigate with methyl bromide (MBr). When MBr was removed growers turned to grafting preferred scions such as an heirloom tomato variety that has no Fusarium wilt resistance onto root stocks with resistance to that disease. The problem is, and it has become yearly that I see it, when we have one of these heavy down pours and the field floods and the flood waters become high enough (or plants are too low in the plant hole) to over-ride the graft on the plants and the scion becomes infected with the disease the root stock is resistant to (Fig. 1).
Another potential problem is when flood waters sit in the field for 48-72+ hrs. and various root rots caused by Rhizoctonia or Pythium species infect the grafted root system which is not resistant to these pathogens. At other times when a tomato plant sits in water-logged soil for days it starts to put out adventitious roots and these roots can develop from the base of the scion when grafted to some rootstocks. These adventitious roots can then come into contact with the soil and introduce soil-borne pathogens into the scion.
The frustrating part for growers in all of this is that this little 2-3-hour event in the 5-month or more production cycle of this crop could disrupt much of the work the grower has put into managing the crop. Bottom line is that grafting is a great way of managing some soil-borne diseases for our vegetable crops and has become very common place as a tool growers can utilize. However, even if growers use grafted tomato or cucurbit plants, they need to understand that they may not be “home-free” from these soil diseases and need to include some practices that help alleviate the possibility of flooded fields. One thing that can be done now is to be sure you have the best drainage for a field so that water from downpours can be moved off of saturated ground more quickly. And it may be practical in the future to increase the size of beds from 1-3 inches to 6+ inches to prevent flood waters from over-topping them.
Figure 1. Row of grafted heirloom tomatoes wilting due to a soil disease in a briefly flooded field.