2017 ASHS Annual Conference

The Lower Rio Grande Valley of Texas is subject to semi-arid temperatures along with high light intensities and periodic drought. These conditions lead to an increased likelihood of sunburn on citrus fruit produced in this region. Depending on the degree of damage, sunburn can affect the internal characteristics of the fruit, such as sugar content, mineral composition, membrane structure and cause skin blemishes. All of these factors can lead to poor fruit quality and decreased packout in post-harvest processing. Sunburn accounts for up to 2% of the wholesale losses as the fresh market is highly dependent upon physical appearance. The prevention of sunburn in citrus fruit is usually addressed through management practices such as: calcium sprays, shade nets, reflectants, and timing sprays to avoid application when temperatures are at their hottest. Products like kaolins and anti-transpirants can also be used to control the plant heat stress and may reduce sunburn in not only fruit, but the leaves and other tissues. In this experiment, we applied two rates of calcium sprays and an anti-transpirant spray to mature grapefruit trees to study their effects on the incidence of sunburn, tree water stress, fruit parameters and yield. Temperature, relative humidity, stem water potential, fruit temperatures, stomatal conductance, sunburn incidence, and fruit quality data were collected to determine the effects of treatments on sunburn and tree stress. Preliminary results showed a correlation between higher fruit temperatures and sunburn incidence in grapefruit. While there was a higher percentage of sunburned fruit on the west side of the tree, the difference was not statistically significant. Sunburn incidence was inconsistent between treatments and thus not conclusive at this point in time. Tree water status was not impacted severely by treatments, as the trees were watered regularly, however, the trees treated with the anti-transpirant showed lower average stomatal conductance. The lowered rate of stomatal conductance may have led to the observed incidence smaller fruit in this treatment. Conversely, calcium treatments had larger fruit and higher yields. Overall, the different treatments showed different effects on the temperatures and stress parameters, but show promise in reducing fruit temperatures and as a result, sunburn in grapefruit grown in Texas.