Causes of Square Abscission in Cotton in Arizona1

Abstract

Cotton (Gossypium hirsutum L.) plantings sometimes abscise large numbers of flower buds (squares). Many environmental and nutritive stresses and insect or disease damage are known to induce this abscission. To determine the specific causes of abscission in Arizona the squares were examined visually under a ✕ 10 stereomicroscope. Symptoms that indicated the cause of the abscission were observed to fall into four categories: 1) dissolution of tissues due to plant bug (primarily Lygus spp.) feeding, 2) soft rot due to thrips (Thysanoptera spp.) activity, 3) tunnels due to lepidopterous larvae feeding, and 4) desiccation without physical damage indicating physiological stresses. Each week during three seasons (1978 to 1980) the abscised squares were subdivided among these categories. Prior to 15 July the primary cause of abscission was Category 1 with a minor fraction due to Categories 2 and 4. After 15 July Category 3 became more important and Category 2 was seldom observed. In only two sampling periods during the 3 year study did Category 4 exceed half of the abscised squares. Correlation of the Category 1 symptoms with lygus bug population in the field at the beginning of the week of sampling was high (r = 0.94 P < 0.05). The regression analysis indicated a feeding rate of 1.7 squares insect−1 per day−1. Temperature correlation on lygus feeding was not significant. The study showed that visual symptoms of damage can be used to categorize abscised squares by the causal agent and that large variation within and between years occurs.