Nightshades are a problem across north central US. The problem in potato is that nightshades are in the same family (Solanaceae) as potato (Solanum tuberosum), as well as tomato, pepper, and tobacco. These weeds harbor many of the same pathogens causing diseases such as early and late blight and pest insects such as Colorado potato beetle and aphids as potato. Most herbicides that affect nightshades also affect potato making control difficult.
Three species of nightshades are common in the north central states. Hairy nightshade (S. sarrachoides) is native to South America and is found coast to coast along the south of US but is also found as far north as eastern Nebraska, eastern S. Dakota and southern Minnesota and Wisconsin. Eastern (black) nightshade (S. ptycanthum) is native to Europe and is found east of the Rocky Mountains. Cuttleaf nightshade (S. triflorum) is native to North America and is found east of the Cascade Mountains. Black nightshade (S. nigrum) is found along the Pacific coast.
Leaf shapes are similar. Hairy nightshade leaves are covered with fine hairs, whereas eastern black nightshade leaves have only a few hairs. The fine hairs on hairy nightshade give the leaf a silvery gray color and may be “sticky” to the touch. Eastern black nightshade leaves are dark green in color and usually have “shot holes” from insect feeding. Cutleaf nightshade leaves are deeply lobed and resemble a holly leaf.
Hairy nightshade is an annual and is usually prostrate in growth habit. Eastern black nightshade may act as an annual or short lived perennial; it may have an erect or spreading growth pattern. The mature deep blue or purple berries are sometimes used for making jams and pies; however, the green immature fruit may be poisonous to man and other animals. Green plant parts and the fruit of nightshade contain toxic glycoalkaloids called solanines which are poisonous. (See Panel Physiological Disorders / Greening.) Numerous cases of black nightshade poisoning have been reported in cattle, sheep, swine, horses, chickens, and ducks. The toxicity of a given nightshade species may vary over wide limits with environment, plant part and degree of maturity affecting toxicity.
Besides being poisonous, the berries present additional problems with harvest and crop quality. Nightshades are frost tolerant and, therefore, stay green into the harvest season. When nightshades are harvested, the green foliage and juice from ruptured berries can foul combine harvesters and may cause harvesting problems.
Nightshades reproduce from seed. Research from Minnesota showed that a single eastern black nightshade plant can produce as many as 7,000 berries and 800,000 seeds when grown without competition. The seed can remain viable in the soil for up to 39 years and can emerge from a depth of two inches. The seeds readily germinate when soil temperatures are between 68 and 100 F. Consequently, nightshade germinates from spring through summer. Those nightshade plants which emerge even in midsummer are capable of producing seed but the amount of seed produced declines as the plants germinate later in the season. Research from Minnesota showed that eastern black nightshade produced less than 85 berries per plant when planted in May, 0 to 3 berries when planted in June, and no berries when planted in July. However, if the canopy is defoliated in July from hail or insect feeding, nightshade planted in May produced up to 1,600 berries per plant. This implies that nightshade have the ability to adapt very quickly to the light made available after crop leaf senescence.
Crop competition is a means of keeping a nightshade problem in check. This is also substantiated by the fact that nightshade plants growing in the row are smaller and produce less berries than those plants growing between rows. Nightshade competes with the crop primarily for water and nutrients. However, competition for light can reduce nightshade problems.
Several cultural practices are valuable in controlling nightshade. Nightshade is easy to control in corn, small grains, and alfalfa; therefore, controlling nightshade in these crops rotated with potatoes can be helpful in reducing weed density. Cultivation can also be a valuable tool in reducing early season nightshade populations since small weeds can easily be controlled with timely cultivation. Growing potatoes in narrow rows or planting varieties with early season vigor will allow the plant to cover the inner row earlier in the season and reduce the competition from nightshade emerging later in the season.
Nightshades may be controlled with herbicides such as EPTC, metaloclor and rimsulfuron among others. None of the herbicides labeled for potatoes will control all of the nightshade present.
Buffalobur (S. rostratum) is another member of the Solanum genus that is found in potato in the prairies of western U.S. especially on sandy soils as in western Nebraska. It is drought-resistant and is a favorite host of the Colorado potato beetle. It is not considered a problem since it does not compete well for nutrients and is often shaded out by potato plants. It grows up to 2.5 ft. Its most notable characteristic is that its stem and branches are totally covered with sharp spines. Flowers are a bright yellow making it easy to spot. As other members of this family, the leaves, berries and roots contain glycoalkaloid toxins that can injure livestock. Native Americans used to make tea from small amounts of ground up roots.
Nightshade leaves and fruit: from left, hairy, cutleaf and eastern black
Fruit of three nightshades
Black nightshade Hairy nightshade