Written by Lenny Wells
Based on conversations I have had with a number of growers it sounds as if there is a need for some clarification regarding phosphite materials. Phosphite has become a very valuable tool for us in the battle against pecan scab. Dr. Tim Brenneman first began testing phosphite materials on pecan primarily for efficacy on some of our minor foliar diseases like anthracnose a number of years ago (since at least 2009).
In the course of Dr. Brenneman’s research he began to notice that phosphite also had a significant effect on scab—especially leaf scab. As his research continued, he developed a large data set on phosphite over several years, using many different phosphite products. UGA Extension recommendations for phosphite use are based solely on Dr. Brenneman’s research. Initially it was only recommended for use in combination with other products, never as a stand-alone. Dr. Brenneman now has the data to show that at rates of at least 2 qts per acre, phosphite can be used alone for leaf scab during the pre-pollination stage. It is an excellent leaf scab material and this is where its best use lies. It can be used during nut sizing as well but in high scab pressure situations, should be tank mixed with another fungicide for nut scab.
Now, what exactly is phosphite? Phosphite (H3 PO3 ) is derived from Phosphorous acid, NOT Phosphoric acid (H3PO4) which is a fertilizer. Phosphorous acid dissociates to form the phosphonate ion (HPO3 2-), also called phosphite. Phosphites are highly systemic and very stable in plants. There is evidence that phosphite may stimulate host defenses .
Phosphite or Phosphorous acid is not converted into phosphate, which is the primary nutrient source of P for plant. There are bacteria capable of transforming phosphite into phosphate, but this process is so slow that it is of no practical relevance. To date, no plant enzymes are known to convert phosphite into phosphate. Therefore, any claims that phosphite can contribute to P nutrient requirements for plant growth should be taken with much caution.
As a matter of fact, phosphite can trick plants into thinking they have enough P, which may potentially contribute to P deficiency when phosphite is over-used.
There is apparently some concern out there about the salts present in many phosphite products. If purity of product is of specific concern to you, then you should use a phosphite product that does not have salts present (bear in mind, that your soil applied potash [KCl] is also a chloride containing salt). However, Dr. Brenneman’s research has shown no negative effect from phosphite products containing salt in his research going back at least 10-11 years. There is no evidence for greater toxicity risk in phosphite products containing salt than for those without salt.
If you are concerned about the long term effects of products containing salt, you may also want to consider the fact that phosphite itself is not metabolized by the plant and we also do not know what the long-term ramifications of that may be. Let me be clear, we do not have any evidence that would lead us to believe this will cause any detrimental effects to pecan trees. Nor do we have any evidence to suggest the salts present in some phosphite products will cause any detrimental effects. If independent research on either topic leads to more information on this, we will let you know.
In short, Dr. Brenneman’s research trials have shown that as far as efficacy is concerned, all the phosphite products Dr. Brenneman has tested (which include most of those we currently use in pecan—I won’t list them because there are too many to be named) work equally well on pecan scab and the other minor diseases from which phosphite provides protection and there is no more risk of toxicity with one phosphite product than there is with another.