University of California Cooperative Extension Ventura County
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Ventura, CA 93003
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The Small Fruit Dilemma
By Nick Sakovich
There are a number of products today that claim to increase fruit size. They range from simple fertilizers to complex fertilizers and special microbial soups. Some of these preparations may work, but many will not. From grower observations, it seems that some of the kelp based products, which contain cytokinins do work. However, this article is not about special products. If you want to increase fruit size, before you spend money on a particular product that may or may not work, there are a few basic cultural practices which a grower must learn. Most of our size problems can be remedied if these cultural practice are properly carried out.
First let me mention a word about the familiar occurrence of alternate bearing. The standard theory behind alternate bearing, simply put, is that when a tree produces a heavy crop one season - and part of that time carrying two crops - it has exhausted much of its energy and must therefore 'rest' the next season by producing less fruit. When there is lots of fruit we know that, in general, those fruit will tend to be small; as opposed to an off year when there are fewer fruit which will then tend to be larger. In lieu of recent research and observations, the depletion of energy theory may or may not be true. Some evidence points to other limiting factors. Regardless of why it happens, until we can solve this problem, we must live with the facts of alternate bearing and small fruit. And, in this case, if we want to increase the size, we simply must decrease the fruit load. This may be accomplished by hand thinning, chemical thinning or pruning (which basically decreases the number of fruit).
Aside from the small fruit sizes associated with alternate bearing, produced during the ‘on’ year, lets now look at the very important cultural practices which the grower must master in order to insure good size fruit.
1. Irrigation The most important of these practices is irrigation. It has been shown, that water stress during the late spring/early summer can cause a yield reduction, otherwise known as June drop (although some drop is normal). Once we get past that period, trees can tolerate substantial water stress without crop loss. However, what will happen if we don't apply adequate water, will be a significant reduction in fruit size. June is critical for fruit drop(yield reduction), and July-Oct is critical for size reduction. This has been documented in research going back to the 1920's.
One can easily under irrigateing during the hot summer months. But growers must also be aware of a potentially warm fall.
Looking back over several years of ET values for the Ventura County area, we see a typical reduction in ET, from August to September of 25%, and likewise from Septemper to October, another 25%. From the peak summer months of July and August, to October, we can typically see a reduction in ET of 50-60%. However, every year is not always typical. In certain locations and in cerain years, ET values have dropped from August to September, not by 25%, but by 5-15%. Likewise from September to Octerber, another 5-15%. Therefore in a given year, instead of the traditional 50 or 60 % drop, a 10-30% drop in ET may occur.
If one is conciencious about irrigation, and irrigates by, lets say tensiometers and perhaps a soil probe - instead of by the calender or a set time year after year - then one has little to worry about. On the other hand, if one reduces his irrigation by 50%, because thats what typically happens with ET values, and we have a particular year where ET is only reduced by 25%, this grower is now under irrigation by 25-35%; and consequently, will see a reduction in fruit size.
2. Phytophthora - With resistant rootstocks, proper irrigation, and at last resort, chemical applications, we have done an excellent job in keeping phytophthora root rot in check. However, and especially in wet years, there is plenty of phytophthora out there. It is not always killing trees, but it can cause enough feeder root destruction to at least weaken the tree. This weakness will come in the form of lack of vigor, some decline in yield and/or a loss in fruit size.
Proper irrigation, resistant rootstocks and fungicide applications are all ways of ensuring that the phytophthora problem be keep at a minimum. Sampling for the fungus can be helpful, but remembering that a negative report does not necessarily mean no phytophthora. It may be that the sample simply did not contain phytophthora, or perhaps it was the wrong time of year for that particular species. By observation and inspection of the tree and its feeder roots, one may wish to treat even though lab results may have been negative.
3. Nematodes - Going hand and hand with the phytophthora fungus is nematodes. High nematode populations can also cause a decrease in fruit size. Growers need to test for nematodes, and, especially if irrigation and phytophthora are under control, consider a nematicide treatment. For a more comprehensive discussiion of nematodes refer to California Grower.
The above, I believe, are the top 3 culprits in our small fruit size dilemma. The following, to some degree, can also be implicated in this problem.
A. Excess nitrogen - Many years ago it was demonstrated that as we increase nitrogen, fruit sizes decreas. As the leaf nitrogen goes up, the number of fruit harvested also goes up, but the volume (after 2.4% leaf nitrogen) remains the same. If numbers increase but volume stays the same, then size must go down. So, as we add nitrogen, more fruit is produced, but since the volume does not increase, the size of the individual fruit goes down.
Give your trees adequate nitrogen but do not over fertilize. The old adage, "if so much is good, then more would be better", does not apply here.
B. Other Pathogens - In addition to phytophthora, there are a number a soil fungi which we may classify as weak pathogens. We might call them 'nibblers', slowly feeding on the trees feeder roots. Especially under stressful conditions, and especially in older orchards, this array of fungi may cause a decrease in tree vigor and fruit size. To a degree that we don't yet quite know, viruses like Tristeza may also be contributing to the fruit size problem. Practically all of our orange trees are on resistant rootstocks. But many of our trees are still infected with the Tristeza virus. Under these conditions, the virus will not kill the tree or even visually damage the tree. But a slight size decrease may occur.
In an older groves in particular, gophers, at one time or another, have probably feed on some of the tree roots. These trees may very well have recovered, but not until a a fungus, perhaps awood rotting fungi, had invaded through this wound. In most cases, the trees are strong enough to wall off the infection. However, as the trees grow older and face different stressful situations, these infections may advance, not enough to seriouly injure the trees, but enough to cause a decline in vigor and a decrease in fruit size.
The McKellar Project, San Joquin Valley - was a major research endeavor in the 1980's, by UC researchers. This projct looked at different application rates of irrigation, fertilization, insecticides, a nematicide, fungicide and a plant growth regulator -- and the interactions of all these. Here is a summary of the results as they relate to fruit size.
a. irrigation - fruit size increased as the irrigation increased from 80 to 100 to 120% ET, each of three years.
b. nitrogen fertilizer - fruit size decreased as nitrogen application levels went from low to medium to high (although not as dramatic as with irrigation).
c. phytophthora/nematodes - size also increased with a fungicide/nematicide application.
There are many reasons for small fruit size. If however, we practice good grove management, and in particular, are timely with irrigation, careful with fertililzation and aware of phytophthora and nematode populations (and treat accordingly), we will have come a long way toward solving the small size dilemma.