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Source: USDA, 1/28/21 Carrots, while in the ground, are
home to a community of microorganisms, some members of which wage a kind of
battle for supremacy against others that cause plant disease and costly losses
to farmers who grow the vegetable.Now, a team of university and
Agricultural Research Service (ARS) scientists has begun to identify some of
these "good guy" microbes. Reported in the June 2020 issue of PLOS One, the findings
provide a critical step toward learning how carrot growers could enlist these
beneficial microbes as an effective defense against diseases like Alternaria
leaf blight. One clue to emerge is
soil-building practices that allow the microbes to colonize carrots, protecting
them from attack. Another tantalizing clue is the genetic makeup of the carrot
varieties themselves, which may predispose them to being colonized. Annually, Americans eat nearly 14
pounds of carrots per person, making it the nation's six-most consumed fresh
vegetable. In addition to adding appealing color and flavor to salads, vegetable
medleys and other dishes, carrots are rich in beta carotene, vitamins, minerals
and dietary fiber. Seven states, led by California, produce 94 percent of the
U.S. carrot crop. Threatening that farm-to-table
supply, however, is Alternaria
dauci, the fungal culprit behind Alternaria leaf blight, which occurs in most areas where carrots are grown. Unchecked, the disease causes decay in both the carrot's distinct feathery leaves above ground and taproot, below ground, diminishing its yield, quality and marketability. Fungicide use, rotations with non-host crops and resistant varieties are among tactics used to counter Alternaria leaf blight. However, the potential of bacteria and fungi that peacefully colonize the carrot plant—called "endophytes"—may also have merit, said Philipp Simon, a plant geneticist who leads the ARS Vegetable Crops Research Unit in Madison, Wisconsin. To learn more, collaborators with
Purdue University and the Organic Seed Alliance, together with Simon, evaluated
36 diverse commercial carrot varieties and breeding lines for their reactions
to Alternaria leaf blight in either conventionally managed or organically farmed
plots. In the conventional plot, for example, pre-emergent herbicides were
applied after the carrots had been planted, while in organic plot, weeds were
hand-pulled as needed. The researchers ranked the
severity of Alternaria leaf blight in the carrots on a scale of 1 to 12 and
harvested the vegetables. They grew cultures of endophytes from nine of
lowest-scoring (healthiest) carrot varieties and identified them using DNA
"fingerprinting" methods. They also conducted petri-dish and seed
experiments confirming the endophytes' anti-Alternaria activity. Among their findings:
- More bacteria (22
species) than fungi (six species) were isolated from the carrot varieties.
- Stenotrophomonas,
Xanthomonas, Pseudomonas, Paenibacillus and Methylobacterium bacteria
were among the most prevalent.
- Soils in organic
plots had more organic matter, a greater diversity and abundance of
endophytes, and plants grown in these plots had lower disease levels than
conventional plots.
- Some carrot
varieties' genetic makeup made them more "endophyte-friendly"
than others, especially in the organic plot. Other varieties hosted lots
of endophytes regardless of how they were grown, suggesting a complexity
that still needs to be teased out.
Exactly how the endophytes
protect carrots also isn't well understood. Some possible explanations include
excluding Alternaria leaf blight fungi from choice space and nutrients,
secreting antibiotics to keep them at bay or by helping mobilize the carrot
plant's own defense, called "induced systemic resistance." The researchers plan additional
studies to learn more about which mechanisms are at work in carrots and explain
these results. They will also conduct more field trials to confirm and expand
upon the results observed thus far. The Agricultural Research Service is
the U.S. Department of Agriculture's chief scientific in-house research agency.
Daily, ARS focuses on solutions to agricultural problems affecting America. Each
dollar invested in agricultural research results in $17 of economic impact.
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