Will the spring drought hurt growers?

Article reproduced with permission
Citrus Industry Magazine, August 2000

By Larry Parsons

The first five months of 2000 were the driest in central Florida since 1915, when records were first available. Trophy-size bass died as some Florida lakes dried up. During the dry spring, forest fires were common, and a number of forest trees died or were weakened because of the drought. Florida has gone through periodic droughts before, but this was one of the worst.

How will this affect citrus growers? For the few who had no irrigation or an inadequate irrigation system, the consequences could be serious. This drought will show the deficiencies in any irrigation system. Most growers use microsprinkler or drip systems, and a plugged emitter will result in a wilted tree. Maintenance is essential; systems need to be checked and lines need to be flushed periodically to minimize emitter plugging.

Florida had a serious spring and summer drought with forest fires in 1998, and orange production decreased by nearly 24 percent in 1998-99 compared to the previous season. Part of that decrease was due to El Nino flooding, but the spring drought was probably a major factor in the reduced fruit production in the 1998-99 season. In spite of irrigation, the citrus crop harvested in 2000-01 after the severe January to mid-June, 2000 drought may also be reduced.

Leaf and fruit growth are some of the most sensitive processes affected by water stress. With inadequate irrigation, new flush and overall tree growth will be reduced. Leaf and fruit expansion will be slowed or stopped. Fruit size will probably be smaller. Wilting and leaf curl will increase. Stomata will be more closed and this will reduce photosynthesis, which produces sugar for fruit solids and tree growth. While there is normally some young fruit drop in May and June, immature fruit drop will increase, and this can lead to an ultimate statewide yield reduction.

Drought does not cause disease, but drought can aggravate some diseases or speed up expression of symptoms. Trees on sour orange infected with decline isolates of citrus tristeza virus (CTV) lose most of their functional fibrous root system and cannot extract as much water from the soil. With blight, the fibrous roots are functional in the early stages, but blockage of the water conducting vessels (xylem) occurs in the trunk and scaffold. Both diseases will develop in the absence of drought, but drought can enhance either disease when the tree is on the threshold of showing symptoms. Hence, drought can speed recognition of stress-related diseases that may have gone unrecognized for a while under normal conditions.

Because of the high temperatures, clear skies with high radiation loads, and low humidities, evapotranspiration (ET) rates were high in May and early June, 2000. ET rates commonly exceeded 0.21 inch/day. With 140 trees per acre, trees need over 40 gallons of water per day to meet that ET demand. Normally, some of that water would have come from the non-irrigated zone, but that has not been the case in this severe drought.

This drought has clearly demonstrated the importance of coverage by the irrigation system. Robert Koo emphasized this in studies using drip, microsprinkler and overhead sprinkler studies in the 1970s and 1980s. Our recent studies have shown that microsprinklers that cover too small an area under the tree cannot meet tree demands, no matter how much water is applied. Much of the time, roots outside the irrigated zone can extract some water from the rain-fed non-irrigated zone. Because there was so little rain, there was essentially no water available to those roots from the non-irrigated zone. Irrigation is normally a supplement to rainfall in Florida, and rain is the primary source of water to meet tree needs. However, in this drought, the irrigation system became the primary supplier of water until the rains returned.

If a microsprinkler system has one jet per tree and 140 trees/acre, a system with 10-foot diameter emitters covers only about 25 percent of the total land area. Roots spread throughout the entire grove floor in a mature grove. With this spray pattern, only a fraction of the total roots in the grove are irrigated. When there is essentially no contribution of water from the non-irrigated zone, it is tempting to run the system for a long time to make up for the lack of rainfall. On the ridge, this would drive the water deeper into the soil. Running the water after the top three feet of soil are at field capacity will move water below the root zone and not benefit the tree. It may make the grower feel better, but irrigating for long durations wastes water by moving it below the main root zone. Since most of the roots are in the top two to three feet, it makes sense to keep
this area wet and not drive water much below it. If possible, daily irrigation for two hours would be more effective in improving water use efficiency than running the system for six hours every three days. Tensiometers or other soil water measuring devices set at six-, 12- and 24-inch depths can determine depth of wetting and help guide irrigation scheduling.

It is possible that even daily watering with a small diameter jet cannot meet the needs of mature trees during drought periods. In that case, the grower needs to change the emitter to increase the spray diameter. It is best to cover the entire area under the canopy and possibly even some of the area beyond the canopy edge. Greater coverage will wet more roots and provide more water for the trees. By having a large wetted volume, the tree¹s water needs will more likely be met during prolonged dry periods.

Summer rains are beginning to make up for some of the prolonged spring water deficit, but it will take many rainy months or a hurricane to make up for the shortfall. This year¹s drought emphasizes the importance of a well-maintained irrigation system with sufficient coverage to meet the needs of the trees. Water conservation practices which result from a drought can only work to help growers meet their future water needs in a high growth state where water, even with normal rainfall, is rapidly becoming a precious commodity.

Larry Parsons is a professor at the UF/IFAS Citrus Research and Education Center at Lake Alfred.

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