Potato plants emerge from the ground 2-6 weeks after planting, depending on weather, location, and time of year. The plants grow quickly, and will begin to grow tubers just a few weeks after emergence.
After the plants emerge, the grower has to be very careful to keep his field thoroughly watered. Potato plants can survive periods of dry soil, but drought-stressed plants produce fewer potatoes of low quality. During the hottest weeks of summer, center pivot machines may run 24 hours a day, watering an entire field every 20-30 hours.
After emergence, growers must carefully keep track of the health and growth of their potato plants. They regularly take leaf samples and have them chemically analyzed for the amounts of essential plant nutrients. When the fertilizer they applied before panting starts to get used up, they see this in their leaf sample tests, and begin applying fertilizer to their growing plants. Toward the middle of the growing season, growers stop all fertilizing. This encourages the plants to put most of their growth into tubers for the second half of the season. Too much fertilizer late in the season causes plants to grow huge above ground, with very small tubers.
Many pest diseases and insects attack potatoes. Growers carefully watch their fields for disease symptoms and insects. Scientists track disease and insect populations throughout Washington’s potato-growing region, and provide helpful information to the growers. One of the most important pests of potatoes is late blight. This is the disease that caused the Irish potato famine of the 1840s. This disease can completely destroy a potato field, and can quickly spread to neighboring fields; everybody in the region must control it. Growers treat their crops to protect them from late blight using fungicides that stop late blight infection. They follow the recommendations of scientists who have studied and understand the biology of the late blight fungus.
The green peach aphid is the most important insect pest for potato growers. It is so important not because it directly damages plants, but because it carries a devastating potato disease called “potato leafroll virus” (like people, potato plants can get sick from viruses – but potato viruses cannot hurt people). This virus weakens potato plants and causes the tubers to have brown internal markings called “net necrosis.” Supermarkets and restaurants will not buy potatoes with net necrosis, so it is very important to growers to prevent it in the field. Similarly to late blight, scientists keep track of aphid numbers throughout the region, and keep the growers informed. When growers find aphids in their field, they have the field treated with insecticides. Some modern aphid-insecticides are incredibly targeted – affecting only aphids and a few close relatives, and are less toxic to humans than table salt. These insecticides preserve beneficial insects in the field, and are extremely safe for workers and consumers.
One important pest that many people will never have heard of is the plant-parasitic nematode. Nematodes are minute worms that infect plant roots and potato tubers. Nematode-infected potatoes are definitely not attractive, and growers cannot sell potatoes infected with nematodes. Many fields must be treated with nematicides to control these pests. Until consumers can accept these unnattractive, nematode-affected potatoes, growers will have to control nematodes.