Silver scurf is one of the most notorious cosmetic diseases of potatoes. Experts have recognized that silver scurf spreads to healthy tubers through the air. Stopping the spread of silver scurf in storage would save millions in fresh potato losses to growers and producers and improve the quality and appeal of potatoes to consumers.
Silver scurf has been found by researchers in soil and on seed potatoes. It is not readily seen in early stages before the population is high enough to show visual evidence of its presence without extensive testing. It can therefore be inadvertently passed from seed potatoes to the commercial grower, where it can inoculate healthy tubers and severely reduce the value of a crop.
Potato storages must be totally enclosed to preserve the temperature and humidity of the storage atmosphere. Since temperature control is essential for preservation, introduction of outdoor air is not available whenever outdoor air is either too hot or too cold.
A serious consideration of the way harmful organisms are spread in the air is beginning to be made in-depth by pathologists and storage experts. Pathogens, particularly fungal spores such as silver scurf, Fusarium and others, are spread by movement in the air. As they sporulate, the spores are released from the host and go airborne, spreading to and infecting other tubers in the storage pile.
This caused growers and producers to look to chemicals for killing pathogens after they have already infected the potatoes. However, it has been demonstrated in tests that a post-harvest application of the sterilant peracetic acid causes increased damage from fungi. It has been concluded that this was probably because the chemical also killed microbials on the surfaces of the potatoes that were antagonistic to the fungi. This is likely to be true of silver scurf and other notorious fungal destroyers of potatoes.
Airborne fungal spores and bacteria can occur in potato storage air in huge numbers, especially when potatoes are being moved into or out of storage. Fungi are a more prominent destroyer of potatoes than bacteria. Silver scurf, black dot, Fusarium, Pythium, early blight and late blight are all caused by fungi. Bacteria, such as Erwinia (soft rot) and ring rot, that affect potatoes are as small as 0.8 of a micrometer. The fungal spores that are most destructive to potatoes are much larger than bacteria, ranging from 4 to 60 micrometers. Silver scurf spores are elongated and are produced on microscopic, string-like stems. The spores will remain fairly still as long as the humidity is kept at 95 percent or higher in the surrounding air.
It is ideal to keep the humidity high in storage to prevent shrink without condensation and wetting the potatoes, plenum floors, walls and other surfaces. The organism requires water to survive and grow, so excess moisture on potatoes and other surfaces will contribute to the development and spread of the disease. The spores are freed from their stems by disturbances such as the physical handling or by forced movement of surrounding air, particularly when the relative humidity is below 85 percent. Like weed seeds, silver scurf and other fungi propagate by spreading spores through the air to a new host. Silver scurf is unique in that it will freely establish itself on healthy potato skins, unlike Fusarium, Pythium and others that require an invasion point caused by cracks, peels or injuries.
Investigations done by Philip Hamm, Dennis Johnson, Jeff Miller, Nora Olsen, and Phillip Nolte—representing Washington State University, Oregon State University and the University of Idaho—have identified silver scurf to be a major contributor to the reduction of potato value. Cosmetic appearance is the biggest problem.
“Probably more important from a cosmetic standpoint are the ‘secondary lesions’ that result from infections that develop while the tubers are in storage,” note the researchers. “Secondary lesions may occur anywhere on the tuber and can be so numerous as to cover virtually the entire tuber surface. In storage, infected tubers lose water at a higher rate than healthy tubers due to the pathogen’s disruption of the periderm.”
Reed Searle, a grower of red and yellow varieties in Shelley, Idaho, experienced heavy losses to silver scurf and black dot disease for several years before Humigators were installed on five of his storage bins in 2012. The diseases have not shown themselves since installation but were still prevalent in the cellars that did not have the machines. Searle claims that the machines paid for themselves within the first year of installation. His experience has led to additional work being done toward verification of the benefits of the removal of airborne spores.
In May 2015 a series of tests were done by Isaacs Hydropermutation Technologies (IHT) of Rigby, Idaho, in cooperation with Allen Floyd and his team at Harvest Fresh Produce in Othello, Wash. The tests were performed at one of Harvest Fresh’s storages where silver scurf had been previously identified. A Model 1225 Humigator produced by IHT was used to vacuum the air surrounding the potato pile in the storage facility and to filter the air with water to intercept microscopic particles, including silver scurf spores. The filtered, humid air was released into the plenum of the ventilation system.
A number of samples were taken of the air before entering the Humigator in the storage area and also in the plenum at the outlet of the Humigator. Water samples were taken periodically from the drain valve of the Humigator’s separation tank with pre-sanitized sample bottles. The air sample spore traps were analyzed by Dixon Information in Salt Lake City. The water samples were analyzed by the Oregon State University laboratory in Hermiston, Ore.
Analysis of the air samples, taken periodically and under varying conditions, showed an 81 percent reduction in target spore count at the exit of the Humigator after a single pass. After 12 hours of continuous circulation through the Humigator, no spores were found in the samples.
Water samples were heavily loaded with all kinds of both organic and inorganic particles, including heavy debris and a large number of the target spore.
These tests have shown that silver scurf is airborne in significant numbers in storage and that it can be intercepted from the air by humigation. Intercepting airborne silver scurf in storage would eliminate the spread of the pathogens to healthy tubers through the air. Humigation technology can significantly increase profit to growers and producers, improve the appearance of potatoes in the marketplace, and increase sales of potatoes.