Bonsai Pumpkin Plants

Jerry Brust, IPM Vegetable Specialist, University of Maryland;

No I am not talking about some new “value added” pumpkin variety. I am referring to pumpkin plants that appear to be normal in the field up to about the third or fourth week and then they suddenly stop growing, while plants around them continue to grow. These ‘bonsai’ plants are not deficient in nutrients or water, but their leaves, at times, will turn yellow at the margins and even necrotic. The plants do not wilt or die — they just sit there never producing a pumpkin fruit. I used to think that these small plants were the result of herbicide carryover from the previous crop, usually a cereal crop. This is probably true when there are large sections of the field with the bonsai plants, but what I have seen in the past and I am seeing this year, as are growers and consultants, are many more scattered bonsai pumpkin plants across a field. These plants are difficult to see at this time of year as they have been covered over by their neighbors. One guess I have (as did one consultant) is that squash bugs are causing these miniature plants in the field. We had exceptionally high numbers of squash bugs this year and many growers had trouble controlling them. When squash bugs feed they inject a toxin into the plant that can cause the plant to wilt (often this wilting is called Anasa wilt, Anasa being the genus name of the squash bug). If the bugs feed at the right stage of growth or in such a way that they do not cause much, if any, wilting but the toxin builds in the plant it is possible that the toxin is slowing the growth of the plant. I have no proof that squash bugs are causing these small plants in the field other than the observation that pumpkin fields where I had good squash bug control had no bonsai plants, but two fields where squash bug was poorly controlled and the bugs fed extensively at the base of the plants had a great deal of the miniature plants. I think the damage is worse to the pumpkin plant when squash bugs are allowed to feed at the base of the plant for extended periods of time than if they feed on the foliage. Growers will need to watch their pumpkin plants closely next year and as they begin to grow being sure to check at the base of the plant and under the plastic for squash bugs.