The mystery disease infecting Pennsylvania alfalfa was handily diagnosed by USDA-ARS plant pathologist Deborah Samac.
Samac, who saw eHay Weekly’s story, "Strange-Looking Alfalfa Baffles Experts," in our May 3 issue, clicked on the provided photo link to Penn State University’s (PSU) Field Crop News.
"I looked at the photos and said, 'I bet that's witches'-broom phytoplasma,' " says Samac, who asked PSU Extension forage agronomist Marvin Hall to send her plant samples. She tentatively identified the disease using photos and the disease description from the Compendium of Alfalfa Diseases. Then she isolated DNA and performed a polymerase chain reaction test at the St. Paul, MN, lab to verify that her diagnosis was correct.
"I feel very familiar with all of the alfalfa diseases right now," laughs Samac, who has spent the past few months editing alfalfa disease photos and text to update the 1990 compendium.
Witches'-broom phytoplasma, also called Candidatus Phytoplasma asteris, is the same organism, or very closely related to it, that causes aster yellows, a disease affecting asters and other plant species, she says. The bacteria live in the phloem of plants – they don’t have cell walls – and travel by leafhoppers to crop hosts, including alfalfa and clovers.
Hall spent part of last weekend researching the disease, which is systemic and will remain in the plants their entire lives, he says. At this point, he's hard-pressed to know how to advise growers on the problem.
"It is fairly rare, at least it has been," Hall says of the disease. "It doesn’t show up all the time, only when the plant shows some stress. So when it was really cool and wet this spring, we had some stress and it caused the plants to show (disease symptoms). Then as soon as we got a couple of warm days, they started growing and grew right out of the symptoms."
The plants were stunted, he says. "We don't know if it will affect yields yet."
No alfalfa varieties have resistance to the disease, Samac believes. In Argentina and some Middle-Eastern countries, the disease causes severe stunting and kills alfalfa. So far in the U.S., Wisconsin is the only other state where the phytoplasma has been identified as infecting alfalfa.
The disease is only transmitted by leafhoppers, so it's important for growers with infected fields to monitor and manage populations this year, particularly if new, uninfected fields are nearby, she says.
For help identifying potential disease problems in alfalfa, contact Samac at 612-625-1243 or email@example.com.