Potato Pests and diseases. Here we will look at the main pests and diseases that affect potato yields in Kenya.
The Irish potato, Solanum tuberosum, is an important staple crop grown in Kenya. Potatoes are extremely good for us! Potatoes are free of fats, sodium and cholesterol. Potatoes are packed with protein, slow release carbohydrates, vitamins and minerals, and health promoting phytonutrients.
Irish potatoes are generally grown in the higher altitude areas, on rain fed land, where they compete favorably with maize production. 90% of potatoes are grown on smallholdings on less than 0.5 acres of land. 800,000 people in Kenya directly benefit from farming potatoes. While the potato income chain supports more than 2.5 million people. Potatoes are therefore important for food and economic security.
Potato Pests & Diseases
Over the years the potato production area has increased while the total farm production has reduced. So, farmers are seeing lower yields, less food and economic return on their farms. This is reason for concern and is being addressed at Government and County Level in our Vision 2030.
Early blight is caused by a fungal pathogen, Alternaria solani. It is common in most potato growing areas around the world. The primary infection is caused by inoculum from host plants (weeds / tomatoes / volunteer plants) or infected plant debris from the previous crop. High humidity and free moisture are required for the spores to germinate and penetrate the plant.
They infect the lower mature leaves of the potato plants causing brown dots that enlarge, become angular and then are easily identified because they contain light and dark concentric bands. Under high disease pressure the lesions join up and the leaves become chlorotic and die. Remaining on the plant. Severe foliar infection will cause reduced yields and low dry matter content in the tubers.
Once in a crop the disease can spread fast as the spores are both wind and water borne and move around in dust, air currents, and water splashes. Alternating wet and dry periods are most conducive to the spread of the disease. The fungus can get infect the tuber at harvest reduce affect quality and storage.
Late blight is caused by the fungus Phytopthora infestans, which infects Solanum species, potatoes, tomatoes, eggplants and a number of weeds and local greens. When conditions are conducive and no measures are taken to control the fungus, it can completely decimate the above ground plant parts and has dramatically devastating effects on crop yields. Late blight was responsible for the Great Potato Famine in Ireland in the 1940’s, causing over a million deaths.
Late blight can affect the crop at any time during the growing season but grows especially fast in cool wet weather. Temperatures of 15-21C in the day, and 10-15C at night and 100% humidity can cause the disease to move fast, killing the plants in a few days. On leaves and stems, pale green to brown spots develop, often with a purplish tinge and a greasy water-soaked look, they grow rapidly until while leaves die / plants wilt in a few days. The damaged plant material has a distinctive rotting smell. In the early morning or during very damp spells the lesions will be coated with a white velvety fungal fuzz. The spores infect the tubers, causing a rot that allows other secondary rots (eg Erwinia) to come in, greatly reducing quality and storage.
The disease can be controlled with proper cultural practices to reduce the initial inoculum and preventative fungicides. Curative control is ineffective. It remains dormant in the dry season.
Bacterial Wilt (brown rot)
Bacterial wilt, caused by the bacteria Ralstonia solanacearum, is one of the most destructive potato diseases. Symptoms include wilting, stunting and yellowing of plants, which finally die. Early symptoms include wilting in the day and recovery overnight. The bacteria gets into the tubers, which may show no external symptoms, but they rot inside, and a bacterial ooze comes out of the eyes and stem end attachment, which soil sticks to.
Infected plants should be removed from the field and destroyed, and ash spread on the area. Ralstonia is spread with infected seed potatoes, soil and irrigation water. These can be tested at our laboratory before you plant. Surface water is a great risk for Ralstonia in Kenya, as there are many farms along waters edge, also there are many weeds that harbor and spread the bacteria but show no symptoms.
Potato cyst nematodes (PCN)
PCN in Kenya is a new and emerging pest that as quickly invaded many potato growing regions. Globodera rostochiensis is the main PCN although Globodera pallida has recently been picked up in one growing region. Depending on the level of PCN in the soil and the health a variety of the crop, PCN can reduce yields by a massive 80%.
PCN cysts persist in the soil for up to 20 years – but the higher the population at the start of your crop cycle the higher the damage to the crop. Rotation, rotation, rotation. PCN levels build up in the soil when potatoes are planted. The highest levels of PCN, and the highest crop damage is seen on soils that are cropped twice a year with potatoes. Even one break crop can reduce levels, but ideally potatoes should be grown in a 4-year rotation with other crops. Buy certified clean potato seed (this may not be possible as the demand for clean seed is far higher than the production)!
PCN moves around in soil, water and plant tissue. Check potato seed for infection and discard infected seed in a deep hole. Scrub potato seed thoroughly on land away from your field – to remove any PCN and soil particles that are attached to the tubers. Wash tractors and other equipment (spades / boots / buckets etc.), away from your fields to remove soil and PCN cysts. As much as possible avoid cross movement of animals and people through the fields. Burn / bury infected plant tissue away from fields. Plant PCN resistant varieties, if available.
Potato Leaf Roll Virus (PLRV)
Potato leaf roll virus causes plants to be stunted and more erect, it can cause the leaves to become upright, roll up length ways, and become chlorotic. Plants with seed borne infections are most severely affected. The virus is transmitted via contaminated seed or via aphids. It can take the aphids several minutes to hours to acquire the virus – but once they have acquired the virus, they are infective for life. Control is via clean seed, aphid control and removal of infected plants.
PLRV affects yield, marketability, and storage, it causes the conversion of starch to sucrose in the tuber – resulting in bad chipping quality.
Potato Virus Y (PVY)
PVY is an aphid-borne virus that causes mild to severe mottling and eventually leaf drop in the leaves. It greatly reduces the yield, tuber size and marketability of the potatoes. Infected seed potatoes are the main cause of spread. Once in the field the PVY has a wide host range (Solanaceae, Amaranthaceae, Chenopodiacaeae – ie tomatoes, eggplant, cape gooseberries, many common local greens and weeds). Aphids pick up the virus within a few seconds of feeding and proceed to transfer to one or two healthy plants before until it feeds off another infected plant. Use clean seed potatoes, control aphids. We can test for PVY in our laboratory.
Potato Tuber moth
The potato tuber moth or worm, Phthorimaea operculella, affects production, reduces quality and increases the risk of other pathogens entering the crop. It feeds on a restricted range of plants (potatoes, tomatoes and tobacco). It attacks plants in the field and in storage. The causal insect is a small brown moth – that is active at dawn and dusk. Moths cannot fly far – main spread is via infected tubers. Monitor numbers with pheromone traps, weed, and get together with your neighbours to plant your potatoes in at the same time.
Good cultural practices to control potato pests and diseases
- Field sanitation – remove all early disease inoculum in field before planting – potato plant residues, volunteer plants, culled tuber piles, residual tubers in soil, weeds.
- Source good clean certified seed. If this is not available, check the quality of the tubers you buy, cut open and discard any with symptoms. Wash tubers away from field to remove soil before planting. If you use own seed, mark the healthiest plants before harvest. Check quality of tubers after harvest, cutting some, if in doubt store in a warm place for 15-20 days and discard rotten ones, wash off field before planting.
- Spray preventative fungicides for late blight – these also work on early blight, rotate active ingredients to prevent fungicide resistance build up.
- 2-3 weeks before digging up – cut stems and leave tubers in soil to allow tuber to mature and skins to thicken – this reduces post-harvest infection. Harvest carefully to reduce damages. When harvesting – remove infected potatoes before storage.
- Proper plant spacing to allow aeration
- Mulching soils can reduce water splashes and transmission of late and early blight
- Good soil health and fertiliser use (have your soil tested and use a proper fertiliser program), healthy plants are more resistant to diseases and insects.
- For PCN, Ralstonia – check soils / water / seed potatoes in a laboratory before planting.
- Irrigation can greatly increase yields. Use drip to reduce splashes and disease spread and check the water source – underground water has less risk of disease transmission. Irrigate midday to allow leaves to dry before the evening.
- Check for aphids / leaf minors / cutworm and spray insecticides if necessary.
- Equipment hygiene – clean all tools / tractors / shoes when going from one field to the next. In the morning start clean – in the newer fields and access older fields last – to avoid moving the inoculum from the older crop to the younger crop.
- Understand the disease pressures in your area and plant resistant varieties.
- Plant potatoes deep (15 cm) and hill up after germination to reduce disease / moth infestation of tubers.
- Soil solarization in field, by placing weighted down clear or black plastic sheeting over damp soil for several weeks can pasteurize the soil and reduce inoculum loads.
- Weeding, many local weeds and greens eg nightshade can harbor these viruses and diseases.
- Rotation, Rotation, Rotation – rotate with none Solanaceae crops eg cabbage. Ralstonia and PCN are very persistent in soils.
- Check the weather forecasts / monitor weather. Increase preventative spraying in wet humid weather.
Please contact [email protected] to learn how to tests your potato seed, soil and water. http://shambaza.com/potato-seeds has a list of certified seed suppliers. https://www.baltoncp.com/amirankenya/wp-content/uploads/sites/9/2018/12/potato.pdf has a nice potato spray program
Ruth Vaughan is the Technical Director at Crop Nutrition Laboratory Services Ltd. (CROPNUTS). Ruth is also a contributing author to Kenya’s leading horticulture magazines such as the HortFresh Journal, HortiNews and Floriculture. Ruth is a great believer in soil health, organic matter, biochar and carbon sequestration as a way to alleviate climate change and increase food security. Loves visiting farmers and seeing all the different farming methods