By Tripti Vashisth and Bikash Adhikari
The Florida peach industry has an advantage of producing fruit in the early-market window before Georgia and South Carolina fruit hits the market.
CHILL HOUR ACCUMULATION
Mild winters and fluctuating temperatures in fall continue to be some of the biggest challenges for peach production in Florida. Peach trees are deciduous; therefore, they need a certain amount of cold weather to become dormant (resting period). When the cold weather requirement for dormancy is fulfilled, the warm spring weather ends the resting period and results in vegetative and floral bud break.
The amount of cold weather needed to end dormancy depends on the peach variety. The University of Florida stone fruit breeding program has developed low-chill peach cultivars that require less than 200 chill hours (chill hour is defined as the number of hours between 32 and 45 degrees), making them suitable for Central and South Florida. However, Florida’s unpredictable winter weather is a concern! In the past four to five years, the required chill hour accumulation needed for uniform bud break was often not achieved until late January.
Figure 1 shows the chill hour accumulation from October 2018 to March 2019 in Polk County (red line). The yellow line shows the historic average of chill hour accumulation. Typically, in Central Florida, 100 chill hours are achieved by early January, resulting in uniform flowering by late January and harvestable fruit in the late-March and April window. However, like last year and the previous few years, the chill hours accumulated by mid-January have been lower than 100.
Peach trees that do not receive the necessary chill hour accumulation can have poor or uneven bud break, sporadic flowering, delayed leaf emergence and poor fruit set. This poses the risk of low productivity, non-marketability and missing the high-return, early-market window.
Hydrogen cyanamide (HC), sold under brand names Dormex® and BudPro™, is a plant growth regulator used worldwide in fruit crops to induce bud break under mild and inconsistent winter conditions. When sprayed on defoliated trees, HC can result in advanced and uniform bud break in many fruit crops such as peach, grape, cherry, apricot, etc.
Previous research in North Florida has shown that when HC is sprayed at rates between 1 and 1.5 percent, it can result in bud break advancement by four to six weeks in peaches and blueberries. Compared to untreated trees, Tropic Beauty peaches treated with HC at 1.2 percent active ingredient in the third week of December resulted in a four-week advancement of floral and vegetative bud break.
The fruit from HC-treated trees ripened uniformly and were of marketable quality. The overall yield from untreated (control) trees was higher because there was prolonged bloom and fruit set throughout the spring; however, these fruits were of low quality and unmarketable.
One of the biggest management decisions with HC is time of application. HC works best when a certain number of chill units have already accumulated and before any bud activity. However, if applied after bud swell, phytotoxicity can occur and damage the flower buds, reducing potential fruit set. HC causes bud break through imposing oxidative stress in the buds, resulting in stimulation of dormant buds. Phytotoxicity causes buds that are already at the swollen stage to burn due to oxidative stress burst. HC is an expensive plant growth regulator. These two factors, cost and application decision, prohibit many growers from using it.
In order to address this situation, researchers conducted a study to evaluate efficacy of HC at a lower rate. This study was done in Central Florida (Polk County) from December 2018 to March 2019 with two peach cultivars: UFBest and UFSun. Three rates of HC (0.25, 0.75 and 1.25 percent) were compared with an untreated control.
All the treatments were applied with a surfactant (Induce at 0.125 percent). The treatments were applied on Dec. 24, 2018, when the pollen grain color was translucent. The number of bud breaks, emerging leaves and flowers were counted from time of application to March 2019. In both cultivars, the lower rates of HC (0.25 and 0.75 percent) were effective in inducing bud break similar to the 1.25 percent treatment.
In HC-treated UFBest, bud break was uniform, synchronized and advanced by two weeks, as compared to the control. In UFSun, all three HC treatments caused the bud break to be uniform, compact and synchronized as compared to the control.
It is interesting to note that HC treatment in Central Florida resulted in bud break two weeks early, and HC treatment in North Florida brought bud break four weeks early, as compared to the control. Table 1 shows the average number of flowers opened in the spring season for all the HC treatments and the control. The lower HC treatments (0.25 and 0.75 percent) resulted in the same number of flowers as compared to the control. However, the 1.25 percent treatment resulted in fewer flowers, which suggests the HC phytotoxicity caused bud abscission.
Overall, the research results suggest that HC can potentially be used at a lower rate than 1.25 percent for uniform and advanced bud break. With a lower rate of HC, the cost of application can be reduced with minimal to no phytotoxic response on the peach tree, thereby increasing the profit margin. The response of a peach tree to HC is cultivar and location specific; therefore, it is highly recommended for growers to test HC on a small scale before applying it to large acreage.
Tripti Vashisth is an assistant professor and Bikash Adhikari is a graduate student at the University of Florida Institute of Food and Agricultural Sciences Citrus Research and Education Center in Lake Alfred.
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