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Controlling disease in storage

 A solution for growers hoping to control the spread of disease in storage is the application of phosphorous acid post-harvest, according to crops specialist Steven Johnson.

Johnson, an extension professor at the University of Maine Cooperative Extension, delivered a presentation on phosphorous acid and its uses in potato post-harvest situations to a packed room at this year’s Manitoba Potato Production Days, held in Brandon, Man., from January 28 to 30.

Johnson’s research for this presentation was concerned with the damage that can occur to tubers during several stages of the harvest and storage process – and the impact this can have on disease control, as damage offers an entry point for pathogens.

“Skinning or wounds provide easy access for two devastating potato tuber pathogens: Phytophthora infestans, the causal agent of late blight, and Phytophthora erythroseptica, the causal agent of pink rot,” Johnson says. The two diseases are responsible for massive losses in potato production systems.

Both pathogens, but especially P. infestans, spread rapidly in the field, but tuber-to-tuber contact can also cause the pathogen to spread during mechanical harvesting and tuber unloading and transfer procedures. But Johnson says a post-harvest phosphorous application may help mitigate this problem.

“Phosphorous acid, sometimes called phosphonate or phosphite, is generally formulated as mono- and di-basic sodium or potassium salts, or an ammonium salt of phosphite,” he explains. Phosphorous acid is often confused with phosphate, but it differs from phosphate in its mode of action. While phosphate is a nutritional source that, when used as fertilizer, quickly assimilates into plant compounds on uptake, phosphorous acid is not a nutritional source, and does not become assimilated into plant compounds.

“Phosphorous acid moves in the phloem and limits the growth of some oomycetes, including P. infestans and P. erythroseptica,” Johnson explains. “Phosphorous acid has also been shown to induce and enhance natural defense reactions in plants, often called systemic acquired resistance or SAR. Phosphorous acid has systemic properties within the plant and tuber, but the exact mode of action of phosphorous acid is not conclusively established.”

Research findings
In a study examining the efficacy of post-harvest applications of phosphorous acid in controlling P. infestans and P. erythroseptica on potatoes headed into storage, treatments applied to the potatoes included an untreated check, and potatoes treated with Agclor 310, Oxidate, Phostrol, ProPhyt, and Rampart, respectively – the latter three being phosphorous acid materials. Rates used, per hundredweight, were 0.48 ounces of Agclor 310, 1.5 ounces of Oxidate, 12.8 ounces of Phostrol, 12.8 and 25.6 ounces of ProPhyt and 12.8 ounces of Rampart. All materials in the study were applied at the rate of 64 ounces per ton of potatoes. The potatoes in the study were handled as they would be during typical harvesting procedures.

The first research question posed by the study was: how long does it take after exposure to inoculum before treatment with phosphorous acid proves ineffective? One and three hours post-exposure were chosen as phosphorous acid application timings, in order to simulate a time frame for potential tuber-to-tuber spread during post-harvest handling. The data showed that phosphorous acid treatments provided complete control over the spread of disease.

A second research question addressed whether phosphorous acid treatments could be applied at lower-than-recommended rates. Studies were performed over two years with the same parameters as for the previous research question. According to Johnson, treatments were applied at full, half or one-quarter of the labeled rates, which is 12.8 ounces per ton.

“Looking at one or three hours after inoculation, late blight control started to break down at one-quarter of the labelled rates of either phosphorous acid,” he says. “In both years, all the phosphorous acid materials provided excellent control when used at the full labelled rate.”

Finally, a third research question examined whether reduced-carrier volumes would prove as effective as full-carrier volumes with the same amount of active ingredient. The same phosphorous acid treatments were applied at full, half or one-quarter of the recommended carrier rate of 64 ounces per ton. “Bear in mind that with 12.8 ounces of phosphorous acid in 16 ounces of carrier, there is not a lot of carrier,” Johnson says.

“As in the reduced phosphorous acid rate trial, late blight control started to break down at one-quarter of the 64-ounce recommended carrier rate.”

Johnson also notes the importance of coverage in applying phosphorous acid treatments. Ideally, potatoes should be treated while they are tumbling onto a transfer belt. Solution applied to the belt aids coverage, he says, but growers should ensure the belt is damp, but not dripping, to aid distributing material.

Also key to application is proper use of all materials. “Improper application of phosphorous acid may result in crop injury, monetary loss or poor disease control,” Johnson says. “It is essential to calibrate application equipment precisely and apply the correct rate of material in the proper volume of carrier.”

Johnson and his team designed a special applicator to address the concern of proper coverage, with nozzles on swivel bodies facing forward and backward, to maximize exposure to treatment.

The bottom line, Johnson says, is that phosphorous acid is extremely effective against P. infestans and P. erythroseptica, and it is an excellent control against tuber-to-tuber spread when applied correctly.