Partial defoliation or even complete loss of plants due to insects, hail, mechanical damage or drought are unavoidable in vegetable crop production. The impact of this type of damage on yields and quality varies with the crop, the severity of damage, the timing of the loss and the number of plants left in the field after the damage event. In a study of the response of squash to varying degrees of defoliation, researchers in the North-Eastern U.S. showed that squash tolerated the loss of up to 33% of the total crop leaf area with no loss in yield … if the damage event occurred prior to fruit set. It did not appear to matter if the damage involved the complete loss of some plants as would occur in the event of mechanical damage or if each plant lost a portion of its total leaf area as would be typical in a hail event or with insect feeding. After fruit set, the crop was less able to tolerate this type of damage. In general, fields with high initial plant populations were better able to tolerate the loss of a few plants or the partial defoliation of the entire field. Partial defoliation had no impact on fruit quality … but did tend to decrease the average fruit size as crop maturity was delayed. In areas with a more limited growing season, crops may not have time to fully overcome the check in growth caused by a defoliation event. However, a crop’s ability to recover from any damage event can be improved if the grower implements appropriate management steps as soon as possible. A light application of N fertilizer, some water and treatment with a protectant fungicide may be all the TLC the crop needs.
Source : Rangarajan, Ingall, Orzolek and Otjen (2003). HortTechnology 13:463-467.