University of Idaho Potato Pathologist Dr. Phillip Wharton and his team operate a research program at the Aberdeen Research & Extension Center that focuses on improving potato health in field and storage environments in order to help the potato industry reduce pre- and post-harvest losses. The goals of the program are to develop sustainable potato disease management practices to help the industry in Idaho deal with increasing disease problems caused by climate change, new potato pathogens and novel strains of existing pathogens that may have developed resistance to commonly used fungicides.
Fungicide resistance screening. Early blight caused by Alternaria solani, and brown leaf spot (caused by A. alternate) are very common diseases of potato found in most potato-growing regions of the U.S. Brown leaf spot is often confused with early blight, as its symptoms are very similar. The pathogens causing these two diseases are closely related and both pathogens are often present on the same diseased plants, which makes it difficult to distinguish how much disease is caused by one pathogen or the other.
Traditionally in Idaho, early blight control has primarily depended on multiple fungicide applications throughout the growing season. The strobilurin fungicides—e.g. pyraclostrobin (Headline) and azoxystrobin (Quadris)—are often favored because they offer broad-spectrum protection against a wide range of fungal diseases.
In the past few years, many Idaho growers reported the failure of the traditionally effective strobilurin fungicides to control early blight. Loss of fungicide efficacy is usually associated with the development of fungicide resistant isolates of a pathogen. Fungicide resistant isolates of A. solani have been reported in Idaho and are common in other potato-growing regions. However, these problems may also be due to misdiagnosis of brown leaf spot as early blight. Unlike A. solani, the brown leaf spot pathogen, A. alternata, is inherently more resistant to strobilurins and has never been well controlled by this class of fungicides.
To determine whether loss of fungicide efficacy was due to the presence of resistant isolates of early blight or the presence of brown leaf spot, the IPC funded a study to investigate the problem. Preliminary disease surveys were carried out in the potato-growing regions of Idaho.
Diseased leaves with symptoms of early blight or brown leaf spot were collected and returned to Aberdeen. Fungal isolates were taken from the lesions on the diseased leaves. These isolates were then identified to determine whether they were A. solani or A. alternata. Once the fungal isolates had been identified, they were tested for resistance to a range of fungicides commonly used in Idaho, including the strobilurins as well as the carboxamide fungicide boscalid (Endura).
Results from the disease survey showed that A. solani was the dominant pathogen causing early blight-like symptoms in Idaho. In all the locations surveyed in southern Idaho, all of the diseased plants collected had typical early blight disease symptoms. None had typical brown leaf spot symptoms, which were only observed on potatoes growing in Bonners Ferry in northern Idaho.
Screening of early blight isolates showed there was widespread resistance to the strobilurins and carboxamide fungicide boscalid. In addition, these results confirmed earlier findings indicating that A. alternate is generally more resistant to the strobilurin fungicides, with up to 50 percent of isolates showing resistance. The discovery of boscalid-resistant A. solani isolates is the first report of resistance to boscalid in A. solani on potatoes in the U.S. and Canada.
Molecular disease diagnostics. Potato blemish diseases are of increasing concern because of the rise in sales of washed potatoes with a high-quality skin appearance for the fresh market. Diseases such as black dot (Colletotrichum coccodes), silver scurf (Helminthosporium solani) and black scurf (Rhizoctonia solani) cause skin blemishes, which can detract for the appearance of the tuber and lead to a decrease in the market value of the crop.
Infection by these pathogens not only affects the quality and yield of potatoes for seed and consumption, but also serves as an important source of inoculum for future crops through the sowing of contaminated seed tubers. No quantitative data are available on the extent of these pathogens on infected seed tubers or on the current distribution of the pathogens in Idaho soils.
An improved understanding is needed of the relationship of seed- and soil-borne inoculum of these pathogens to tuber infection, together with data concerning the quantitative contribution of these sources of inoculum to the incidence of disease in storage. This will permit accurate evaluations of the benefits of ‘clean’ seed and define the maximum levels of the pathogen on seed and in soil for the production of table stock crops of the required quality.
With the aid of IPC funding, Wharton’s group is currently developing diagnostic assays based on the polymerase chain reaction (PCR). These assays are required to detect and quantify these pathogens on potatoes and in soil. PCR is a diagnostic method widely used for the detection of plant pathogenic fungi. These assays are highly accurate and can detect as few as three spores of C. coccodes per gram of soil. They are also able to easily distinguish between black dot and silver scurf from infected tubers. Furthermore, results from the tests can be obtained in less than 12 hours.
Other Research Activities. As well as looking at conventional methods for control of potato diseases, the Wharton lab is also studying novel methods for control of potato diseases, such as the use of antifungal plant volatiles and the use of disease forecasting based on accurate weather forecasts to predict optimal conditions for disease development. Such methods help to reduce the grower’s reliance on and cut down the use of traditional fungicides. This reduces the chances of fungicide resistance developing in pathogen populations and prolongs the life of currently effective fungicides.
The potato pathology lab at Aberdeen also provides a diagnostic service for all kinds of potato diseases. Walk-ins are welcome and personnel can be contacted for field or storage visits if needed. Call Dr. Wharton if you have diseased plants in your field or diseased tubers you need to bring in. Input from the Idaho potato industry is also always welcome. If you have questions or comments, feel free to contact Dr. Wharton at (208) 397-4181 or email him at firstname.lastname@example.org.