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Detecting Late Blight Disease of Potatoes

New test paves the way to more rapid diagnoses

"Late blight" gained notoriety in the mid 1800s as the cause of the Irish potato famine. Today this disease is still a force to be reckoned with by both potato and tomato growers worldwide. Part of the problem is the difficulty of detecting it.

Extension educator Carol MacNeil points out that "The BLIGHT ALERT warning system that we recommend to growers certainly gives them an awareness of weather conditions that are conducive to late blight, so they can be watching for symptoms. But the next step--testing sick plants to discern whether they're infected with the late blight fungus--needs to be done more quickly than it's now being done. Precious time can be lost while we wait for the results of incubation." The standard incubation test can only diagnose late blight once fungal spores have developed. Furthermore, the test can be foiled by the presence of other microorganisms that can mask the late blight fungus.

Enter a new technology called polymerase chain reaction (PCR). Cornell Extension Associate Diane Karasevicz explains how PCR works: "Small amounts of DNA that are specific to certain target organisms, such as the fungus that causes late blight, are amplified to detectable levels. If the late blight pathogen is present in the sample tissue, we can tell it by the type of DNA that is amplified." The advantages of PCR are that it can be done very quickly, it can detect the late blight fungus in the absence of spores, and it can sort out the late blight fungus from other microorganisms that may be present.

Previous to the 1997 growing season PCR had proven itself in the laboratory setting, but more field testing was needed. In 1997 Karasevicz evaluated 56 potato and tomato samples using PCR. For 16 of the 56 samples, PCR failed to diagnose late blight even though a cross-check using the standard test showed its presence. Technical problems responsible for these inaccuracies have been addressed, and further research will assess the validity of the test.

Accurate diagnoses of the disease can assist growers in using healthier seed for planting, determining whether crops will need treatment, and evaluating the health of harvested potatoes and tomatoes. Rapid diagnoses will enable growers to stop the spread of the disease before entire crops are lost. Fine-tuning of the PCR test may make such diagnoses a 21st-century reality.