Within commercial potato production, botrytis is a common problematic fungal disease. Also referred to as gray mold, the foliar disease is caused by the fungus Botrytis cinerea and can be found in most potato-growing regions, including the Pacific Northwest.
Relevant: 75-Day IPM Potato Program
“Botrytis overwinters in one of two ways. It can survive as mycelia on plant trash or as sclerotia, which are compact masses of mycelium,” says Kelly Luff, principal scientist for Bayer Crop Science working across the Snake River Plain in southern Idaho. “When the infection starts, it’s just a little necrotic lesion on the edge of the potato leaf, but as long as conditions are favorable, it can spread and actually take down leaflets, petioles, stems and even entire plants.”
The loss of photosynthetic capability can limit tuber yield, says Jeff Miller, president and CEO of Miller Research based in Rupert, Idaho. “When compounded with the effects of other uncontrolled foliar diseases such as early blight or brown spot, growers can easily lose 10% of their yield,” he adds.
Botrytis development is favored by hot, humid environments and the presence of decaying plant material. Because the growing point is located at the top of the potato plant, older leaves lower in the canopy die and drop to the ground as it grows, Miller explains. This provides botrytis spores a place to land and grow.
There are steps that potato growers can take to reduce the impacts of botrytis. First, growers should avoid over-fertilization, especially later in the season.
“Excessive vine growth and a dense canopy can create an environment favorable for botrytis,” Miller says. “Sometimes, growers equate a greener field to success, but they don’t get paid to grow vines. You don’t want vine growth at the expense of tuber growth.”
Excessive irrigation also can exacerbate the spread of botrytis.
“Growers need to make sure they’re meeting the crop’s consumptive water needs,” Luff says. “However, by overwatering, they’re just providing more opportunity for fungal diseases like botrytis to grow. With lush growth especially, you don’t have much air movement down in the canopy, and that really is conducive to disease development.”
Both Luff and Miller recommend that growers subscribe to a preventive fungicide program using products containing pyrimethanil, a Group 9 fungicide. These include Scala® and Luna Tranquility®, available from Bayer.
“Rescue-type approaches aren’t really that effective,” Miller explains. “I liken the use of fungicide to sunscreen. If you put on sunscreen before you go outside, you don’t get burned. But if you wait until you’ve been outside a while, you’ll feel that burn start. Putting sunscreen on then won’t stop the sunburn. The damage is already done."
“It’s the same with fungicide: You apply it to your crop so that you don’t get burned.” Miller recommends that growers apply Luna Tranquility twice — once at row closure and again roughly two weeks later. “We have found that additional applications don’t seem to improve control,” he says, adding that for every dollar a fungicide program might cost, growers could realize a $4-$5 return by controlling fungal diseases.
For more information about botrytis control options as well as other potato pests, refer to resources in the 75-Day IPM Program for potatoes so you can create an agronomic force field.
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