Causes of Potato Leaves Having Dark Spots

Many different causes may lead to dark spots on potato leaves, including fungal and viral infections and pest damage. In some cases, more than one infection may affect plants at the same time, making identification difficult or confusing. Because several potato plant diseases can cause significant losses of plants and food crops, early detection may be critical to stopping the progress of an infection.

Fungal Infections

Early blight, caused by Alternaria solani, leads to brown or black lesions on potato leaves. These spots usually appear on older leaves first and grow in size to develop a bull's eye pattern. The infection may spread to younger leaves, and affected plant leaves may die and fall away. Growers should aim for producing the healthiest plants possible, as strong plants better fight off infection. Fungicides are available, as are resistant potato varieties.

Symptoms of late blight, caused by Phytophthora infestans, include dark green, water-soaked spots on potato plant leaves. Lower leaves are often affected first, and leaves may brown or shrivel as the disease progresses. Late blight can take as little as two weeks to completely defoliate plants. Many fungicide treatments are available to treat severe infections, but gardeners can prevent infections by spacing plants to allow for good air circulation and keeping the foliage of plants dry. Growers should remove all potentially infected plant material from the garden and should not compost infected plants.

Viral Diseases

Potato leafroll virus (PLRV) is spread by feeding aphids. The lower leaves of infected potato plants show the first signs of illness. The leaf edges turn red or purple, and the leaves develop a rolled appearance. When growers plant an infected seed potato, leading to an infected plant, the leaves may appear leathery or die along the margins. Younger leaves turn pale and curl or roll.

Potato virus Y (PVY) spreads by means of aphids and humans. Plants with the virus present with wrinkled leaves or dead patches or rings on the leaves. Affected leaves may die. Growers should plant resistant varieties to prevent outbreaks. Insecticides to reduce aphid populations and certified seed are available to prevent and control both of these viruses.

Pest Problems

Potato leafhoppers are 1/8-inch-long, greenish-yellow insects that feed on potato plant juices. The insects feed from beneath the leaves, making them easy to miss, but the adults may be seen jumping or flying from plants. The main signs of leafhopper damage appear as triangle-shaped brown or yellow patches on the leaf tips called "hopperburn." Plant leaves may roll or curl and look as if they were burnt. The Purdue University Extension warns growers to monitor potato plants for the presence of nymphs starting in late spring, as insecticides must be applied before damage to the plant becomes obvious or potato yields will be compromised.

Complicating Factors

Many diseases of potatoes are also common to tomato plants, peppers and weeds. These plants can pass infection back and forth or introduce infective organisms into the soil. Rain, wind and humans performing regular garden chores can spread infection. Growers should purchase disease-free seed potatoes and select resistant varieties to start their gardens whenever possible to protect their crops from potential damage.

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