...AND MORE RAIN
C-PROBES
PHYSIOLOGICAL IMPLICATIONS
CITRUS RUST MITES
FRUIT SIZE AND SHAPE

...AND MORE RAIN-

June is not supposed to be one of our rainiest months, but this one has been a "frog strangler" across the Valley and much of the state. Normally, this much rain in June might come from some sort of tropical storm system-but not so this month, and I haven't even tried to keep up with whatever meteorological phenomenon is responsible for it. The rain is always welcome to most of us, but it did interrupt the harvest of grain sorghum.

Obviously, the groves are benefiting from the rain and should respond with a good summer growth flush. More than enough rain fell in most parts of the Valley to provide excellent leaching of any accumulated salts in the root zone. After all, the amount of rain probably exceeded the soil moisture capacity of the root zone of citrus trees.

Reservoir levels have benefited as well, in part from inflows but also from the lack of withdrawals for irrigation, topping the 70 percent level for U.S. waters. As you might recall, a lot of water used in the last few weeks was so-called "free pumping" water that was being released from Mexico's impoundments on the Rio San Juan, which enters the Rio Grande near Rio Grande City. Those releases were to lower water levels in preparation for the hurricane season. Inasmuch as a lot of the thunderstorm activity moved sort of northeasterly from northern Mexico across the Valley and South Texas, I would expect runoff into the Rio San Juan reservoirs to be substantial, so don't be surprised if "free pumping" continues.

C-PROBES-

As I mentioned last month, I installed a couple of C-Probe soil moisture sensor devices in citrus. I believe that Rio Queen also installed a couple of the units. Although I do not completely understand all the results I see, I am learning.

One of the benefits of all the rain in late June is that there is no doubt as to the maximum soil moisture capacity of the two soils at the various sensor depths, as it is fairly easy to follow the downward movement of water through the soil profile. With that value determined, it will be easy to establish the point of soil moisture depletion at a given depth that should trigger the need for irrigation.

I do not have to go to the groves to see if it rained-I can do that on-line now. While I cannot tell how many inches of rain fell, I can tell if the rain was substantial. I suspect that it will be possible to calibrate the data to calculate the actual rainfall.

PHYSIOLOGICAL IMPLICATIONS-

As one looks at the graphic data displays from the soil moisture sensors, the patterns of citrus water use become apparent and it may not agree with what you think is the case. Generally, during the five weeks in which the sensors have been operational, soil moisture at the 4-inch depth is depleted more or less steadily from late afternoon until early morning. From early morning until early afternoon, there is no depletion of soil moisture at the 4-inch depth; from then until late afternoon, soil moisture actually increases.

The pattern at the 8-inch depth is similar, except that the start of depletion is delayed about 4 to 5 hours and continues a couple of hours longer than that at the 4-inch depth. Similarly, in one of the soils, depletion at the12-inch depth starts about 4 to 5 hours later and continues another couple of hours longer than that at the 8-inch depth (there is very little discernible depletion at the 12-inch depth in the other soil).

In other words, as the roots extract water from the 4-inch depth, water begins to move upward from the 8-inch depth to replace it. The moisture leaving the 8-inch depth is in turn replenished by the upward movement of water from the 12-inch depth. Obviously, water must also be moving up from even lower depths to replenish that which moved out of the 12-inch depth.

Under the conditions of irrigation and rainfall for the last 5 weeks, there is no apparent depletion of soil moisture at the 20 inch and 36-inch depths-in these two soils as the graphs are essentially flat-line except for the temporary increases due to the heavy rains.

One aspect of citrus tree physiology that is suggested by the data at the 4-inch depth is that when it gets really hot, the tree basically shuts down in terms of root absorption. That?s the time during the afternoon in which soil moisture actually increases at the 4-inch depth. Too, the apparent absence of depletion during the morning suggests that the tree is just idling along.

Given that photosynthesis occurs only in daylight and it is a process that requires water, I'm inclined to think that there must be a time lag of several hours between root uptake of water and its use in photosynthesis, i.e., root absorption of water mostly occurs at night (as that is when soil moisture is being depleted) for use in the next day's photosynthesis (when little or no soil moisture depletion is occurring).

By this reasoning, incipient wilt, which occurs in the afternoon, suggests that the plant has used up most of the water that was in the pipeline from the night before. Absorption during the following night partially restores the water supply within the tree, as wilting is not present the next morning. As water stress worsens, wilting occurs earlier each day and persists longer into the night-ultimately to the point that the leaves are still wilted at daybreak, a situation that many of us saw during the summer of 1998 when there was neither rain nor irrigation to replenish soil moisture.

CITRUS RUST MITES-

Things were going along pretty well in terms of rust mite populations, but the weather conditions of the last two weeks and at present are sure to change that. I have noted some damage, but I believe it to have resulted from poor control earlier (it rained within a few hours of the miticide application). Even if you had good control up until the rains, I would intensify monitoring efforts now, as whatever residual controls you had may have been depleted by the intense rainfall.

FRUIT SIZE AND SHAPE-

Even though bloom was later than normal, fruit size is running about normal to larger than normal. The latter is especially true where there is a lighter set. Fruit shape of Rio Red grapefruit was markedly ovate or elongated at the beginning of June, suggesting a strong tendency towards sheepnosing. Looking in the same groves over the last couple of days, the fruit appears to be a bit more rounded than it was earlier. Will the latter continue or will sheepnosing be a major problem this year? Frankly, I still lean towards sheepnosing, mainly because of the late bloom. Once it is predisposed to sheepnose, the fruit isn't going to grow out of it-the question then becomes one of degree (severity) of sheepnosing.

JULIAN W. SAULS, Ph.D.
Professor & Extension Horticulturist
2401 East Highway 83
Weslaco TX 78596