The question and answer period at information days is always revealing. That’s when growers signal they are keen to know more.

The subject of phosphorous acid has been on potato agendas for several years now, but the management of this product continues to evolve. Andy Robinson, extension potato agronomist for North Dakota State University addressed this topic at the recent Manitoba Production Days in Brandon and received plenty of questions afterward.

Phosphorous acid is applied to protect potato crops from pink rot and late blight tuber rot. It can be applied as a foliar treatment as well as direct on tubers going into storage. However, when phosphorous acid is applied as a foliar, growers often report leaf burning especially if the crop is under moisture, fertility or disease stress.

Researchers such as Neil Gudmestad, Steve Johnson and others have reported the product to be highly effective at controlling late blight and pink rot. Digging into the trial work, additional data shows benefits in controlling silver scurf in storage.  It is not effective against soft rots, Pythium leak and Fusarium.

Part of the problem, says Robinson, is that there are many application use patterns –chemigation, aerial, ground – that result in variation of application. One of the questions is whether adjuvants can reduce injury from phosphoric acid. 

After several trials in the last two years, Robinson concludes that the product can be applied earlier than dime-sized tubers with higher water carrier volumes.  He cautions, however, that this work is ongoing and further research will validate the results from the first year’s trial. To reduce leaf burn, use lower rates of 2.4 to 3.3 litres per acre. Silicone surfactants may help but don’t apply a foliar fungicide for two to three days after using a silicone surfactant because these surfactants will prevent fungicides from sticking to the leaves.

When storage tuber testing was complete, Robinson recorded that with air and air plus chemigation treatments, there was more phosphorous acid in the tubers tested.  Chemigation alone had less phosphorous acid in the tubers tested. Robinson notes that other research in avocados suggests 100 ppm is needed for Phythophtora spp. control.

In summary, Robinson recommends that three or more applications of phosphoric acid are best for pink rot control. Timing is important. Surfactant and silicone adjuvants can help reduce injury when phosphoric acid rates were low, that is less than 2.4 litres per acre. 

Multiple applications of low amounts of phosphoric acid at the right time with a surfactant/silicone will provide the least injury and best protection from pink rot.