If you see yellow leaves on your potato plants, it's a good idea to investigate further.
The problem may be a lack of moisture during hot weather, insects, diseases, or overspray from herbicides-but yellow leaves could also indicate that your potatoes are mature and ready to remove from the ground.
Potato plants need healthy leaves to produce a bountiful crop.
Six Reasons Potato Leaves Are Yellowing
1. Beetles, Aphids, and Leafhoppers
Chewing pests like blister, Colorado potato, and flea beetles create holes in leaves and with enough damage, the leaves can turn yellow.
Sap-sucking pests like green peach and potato aphids cause discoloration and deformation that leads to yellow leaves.
The potato leafhopper extracts sap from the leaves' undersides causing yellowing of leaf tips and margins known as "hopperburn."
2. Fungal Diseases
- Early blight, caused by the fungus Alternaria solani, begins as dark circular spots on potato leaves as the plant matures. If untreated, it destroys the leaves and greatly reduces the tuber production.
- Late Blight, also caused by a fungus, is common if the growing season is cool and wet. The potato leaves will develop dark spots that turn black or yellow and drop.
- Fusarium Wilt and Verticillium Wilt, are difficult to distinguish in the home garden but both cause yellowing, drooping, and curling downward of the lower leaves on a potato plant.
3. Viruses
Mosaic virus, causes the potato leaves to appear mottled with light green and yellow. The leaves will eventually curl and the plant will die.
4. Overwatering or Underwatering
Potato plants are sensitive to moisture stress caused by overwatering or underwatering. They need moist soil to produce high yields and good quality potatoes but overwatering can cause the tubers to rot and the leaves to yellow.
The roots become waterlogged and can't pass the moisture to the leaves. Underwatering causes the plant to struggle to move sufficient water up through the vascular system and the leaves turn yellow and drop.
5. Soil Nutrients
Potatoes grow best in moderately acidic soil with a pH between 4.8 and 5.4. After a soil test to check the pH and nutrient levels of your soil, apply a balanced NPK fertilizer (10-10-10) before planting.
An additional application should be done when the potato plants are about two feet in diameter. A lack of nutrients can cause the leaves to turn yellow and reduce the harvest.
6. Mature Plant
The leaves on a potato plant will turn yellow or brown as the plant reaches maturity. Potatoes average 100 to 120 days to maturity, and the tubers should be harvested once the vines die.
How to Help Restore Green in Potato Leaves
1. Insect Control
If caught early, insecticidal soaps and oils can rid potato leaves of aphids and beetles. There are also commercial insecticides containing pyrethrins that can control pests.
2. Disease Control
Early and late blight are best controlled by spraying the plants with a commercial chlorothalonil fungicide.
Unfortunately, there is no treatment for fungal wilt or mosaic virus. Remove and discard diseased plants but do not compost these plants, as the pathogens spread via compost.
3. Water Correctly
Potato plants need consistently moist soil, but never waterlogged roots. Monitor the weather conditions and adjust irrigation accordingly to provide one to two inches of water per week.
Heavier soil needs watering at least once every five to seven days, but sandy soils may need more frequent irrigation.
When the plants begin to yellow and the lower leaves begin to die, reduce the amount of water so the tubers won't rot.
4. Provide Nutrition
Potatoes should be planted in nutrient-rich soil and additional nutrients added during the growing season.
Five Ways to Keep Potato Leaves as Green as Can Be
- Water correctly. Keep the soil consistently moist but not soggy.
- Plant wisely. Do not plant potatoes and tomatoes near each other because they are affected by the same fungal diseases.
- Provide sufficient nutrients. Test the garden soil and add organic compost or a balanced NPK fertilizer.
- Be observant. Treat plants infected with harmful insects to prevent spread and remove diseased plants from the garden.
- Clean the garden. To prevent problems for next year, remove potato plant debris from the garden after harvesting.
