November 27, 2017
Show all

POTATO FARMING, Disease Symptoms and Management.

IRISH POTATOES, Common Disease Symptoms Management.

compiled by Charity Kibichia,


Irish potatoes grow well in areas that receive an abundant rainfall of 900-1400mm per annum and have a cool climate with temperatures ranging from 20-230C.

Soil type

Irish potatoes will do well in sandy-loamy soils; they prefer well drained loose soils which are rich in organic matter. This allows maximum tuber development. Poorly drained soils often cause poor stands and low yields. Heavy soils can cause tubers to be small and rough.

Soil pH

Potatoes are more tolerant to acidic soils than most other crops. They prefer slightly acidic soils pH range of 5.2-6.5.  High pH promotes the development of Common Scab. On soils with very high pH use of acidifying fertilizers is beneficial.


Nitrogen (N) in excess delays tuber initiation and maturity leading to excessive growth at the expense of tuber growth. Nitrogen should be supplied adequately in early season to support vegetative growth but excessive N later in the season will suppress tuber initiation and reduce yields.

In general, early maturing varieties and those grown for early markets require less Nitrogen (N) than late maturing varieties.

Potassium (K) reduces discoloration of raw tuber and after-cooking blackening. It also improves resistance to harvesting/handling damage and improves storability by allowing tubers to mature fully. In addition, it minimizes the content of reducing sugars thereby ensuring that tubers are better suited for processing into chips and crisps.

K deficiency decreases photosynthesis, thereby reducing starch formation for tuber yield. Deficiency leads to increased incidence of internal black spot bruising and hollow heart disorder (this is browning of the center of the potato).

Phosphorus (P) should be available during early vegetative development and the entire period of tuber growth for  maximum potato yield.

 Lisha Bio-Organic Fertilizer   is recommended for both planting and top dressing. It provides essential plant nutrients (Nitrogen, Phosphorus and Potassium and micro-nutrients), organic matter and beneficial microorganisms to the soil.

Soil testing is essential for building sustainable soil fertility and fertilizer management program for your farm, to increase your crop yields and reduce your input costs.

We recommend the  Cropnuts Laboratory Services  comprehensive soil testing services dubbed Daktari wa Udongo which give customized precise soil fertility programs and fertilizer application reports.

Bacterial Wilt

The bacterial wilt pathogen can survive in the  soil for several seasons without a host. It also survives in water, seed tubers and potato plant remains.

The disease spreads through infected seed, air, water, soil, farming tools, livestock and people . It can transmit from one field to another or from plant to plant within the same field .


  • The disease can cause rapid wilting and death of the entire plant without any yellowing or spotting of leaves. In rapid disease development, all branches wilt at about the same time.
  • The wilting may also start at the tip of the leaves or where the stems branch out spreading to all other parts of the plant.
  • Sections of diseased plants may wilt completely and dry up, while the remainder of plants appear healthy.
  • The leaves of the plant turn yellow at their bases.
  • The growth of the plant is stunted.
  • When the stem is cut there is a dark brown colour in the inner section.
  • When a tuber is cut in half, black or brown rings will be visible. These rings exude a thick white fluid when squeezed or left for a while.
  • Fluid may also be coming out of the tuber eyes. This can be signified by soil sticking to tuber eyes when crops are harvested. Serious infection causes tubers to rot.


Symptoms of bacterial wilt in potatoes.  (


  • Source seed from credible certified producers. The National Potato Council of Kenya potato variety catalogue has a list of credible potato seed suppliers for different potato varieties in Kenya.
  • Clean tools thoroughly before and after use.
  • Rotate with crops that are not related to potato, such as maize or beans. Do not use tomato, pepper or eggplant as they can also be infected and carry this disease.
  • Uproot all infected plants and tubers, with the surrounding soil, and put them in a 2- feet deep pit and cover with clean soil, or burn them.

  • Do not put diseased plants and tubers on your compost heap.

  • Ensure that farmyard materials and manure are fully composted to avoid spread of disease.
  • The plants next to the diseased plants should be harvested only for home use, not for seed.
  • Avoid planting in low-lying or water-logged areas.
  • Remove any potato plant from the previous season while weeding because it may be carrying the disease.

Late Blight.

Late blight may attack any time after the plants get foliage, it is caused by a fungus called Phytopthora infestans.

Proper spacing and drainage reduces the multiplication of fungal pathogens.


  • Initially the lower leaves get water soaked lesions, especially near the leaf margins.
  • When/ if it is humid and weather is cloudy, the lesions enlarge and spread fast.
  • On the underside of the leaves the fungus is observed as whitish growth.
  • The lesions turn black or dark brown as the leaves start rotting.
  • Lesions on the stem appear as black strips along the length of the stem.
  • The tubers have purplish-brown lesions on the surface.
  • On cutting the tuber, a reddish brown dry rotted tissue extends about 1 inch into the tuber.

Symptoms of late blight in potatoes (


  • Monitor fields twice per week for fungal infection since disease development is favored by cool moist weather.
  • The use of protectant or contact fungicides such as Infinito fungicide. Make sure that fungicide is applied to both leaves and stem.
  • Practice good field sanitation by disposing of all cull potatoes after harvesting.

Early Blight.

Early blight is also known as target spot. It affects older leaves first and is rarely found on young vigorously growing plants.  It is caused by a fungus known as Alternaria solani.

Early Blight and Late Blight can occur at the same time in the field, the terms ‘early’ and ‘late’ refer to the relative time of their appearance in the field. Early blight thrives in warm temperatures and humid conditions.


  •  Appearance of brown lesions first on older, lower leaves which then spread up toward new growth.
  • The lesions are small (1-2 mm), dry, and papery and may develop to brown-black  circular to oval areas and characteristic dark concentric rings of raised and necrotic tissue.
  • The leaf tissue often turns chlorotic (yellow) at the edge of the lesion. As the disease progresses, the entire leaf can become chlorotic and then necrotic (brown).
  • The tubers get dark, circular to irregular spots.
  • The sunken lesions on the tubers are often surrounded by a purplish raised border.
  • The tissue under the lesions is dry, leathery and brown.
  • Lesions will increase in size in storage, though they will remain superficial.

Symptoms of early blight in potatoes



  • Planting disease resistant varieties, late maturing varieties are more resistant than early maturing varieties.
  • Avoiding overhead irrigation, water at the base of plant and in the morning.
  • Keeping plants healthy through proper watering and fertilizer application make them resistant to the disease.
  • As much as possible practice crop rotation with plants that do not share the disease. Plants that share the disease include tomatoes, pepper, eggplant and other solanaceous plants.
  • Avoid bruising seed tubers and the mature tubers when harvesting.
  • Several foliar fungicides are registered for early blight. Mancozeb, chlorothalonil and copper are effective against early blight when applied at approximately 7-10 days intervals. Spraying should commence at the first sign of disease or immediately after bloom.

Verticillium Wilt.

Verticillium wilt is mainly soil-born. It can be caused by two different soil-borne fungi Verticillium albo-atrum or Verticillium dahliae, which build up in the soil with repeated potato production and can survive there for long periods. The pathogens can also be carried via seed tubers.

Transmission is through movement of soil through farm machinery, footwear, animals, water and wind. It can also be harbored by other crop species without visible disease symptoms.

The fungus penetrates plants through the roots and spreads upwards in the vascular tissue restricting water uptake and infecting stems, petioles and leaves.

Not all vines that arise from a single seed piece become infected, the symptoms can be seen in patches within the field. This is a differentiating characteristic from plants suffering from drought which wilt uniformly throughout the field.

  • Usually after flowering, the lower leaves begin to yellow in the areas between leaf veins.
  • The symptoms then move upwards towards the younger leaves.
  • Leaf edges and areas between veins turn yellow and then brown.
  • The stems usually remain upright even as the leaves wilt, a grey to brown discoloration can be seen in the lower stem when cut lengthwise.
  • As it is possible to easily confuse the symptoms of this disease with other causes and early maturity, it is important to carry out plant and soil pathogen analysis for conclusive determination. (see  Cropnuts Laboratory Services).

Split stem showing discoloration by verticillium (


  • Carrying out soil analysis to detect the level of Verticillium infestation in the soil.
  • Scouting for Verticillium wilt before rows close. Fields should be monitored at least twice a week. Check wilted plants for brown discoloration of the vascular area of stems.
  • Practice crop rotation by alternating potatoes with non-susceptible cereals crops.
  • Plant certified seed tubers, selecting cultivars that are resistant to wilt. Please see the National Potato Council of Kenya potato variety catalogue.
  • Control host weeds, dispose of infected crop debris,
  • maintain high fertility and avoid over-irrigation.

Bacterial Soft Rot.

Soft rot is characterized by serious rotting of potatoes in storage accompanied by foul odors.

Tubers harvested from plants that were infected by blackleg during the growing season also develop Soft rot during storage.

Blackleg is caused by Erwinia atroseptica and soft rot is caused by E. carotovora. These bacteria can live in soil, in decaying plant debris, and in seed tubers.  Infected vines release bacteria to the soil and the bacteria can move to new tubers through soil water. Soft rot, however, can also enter tubers through the stem end and wounds caused mechanically or through other infections.
Soft rot occurs as a slimy rot of tubers. The stems of affected plants are blackened and rotted at the base of the stem near the soil surface.
  • Sprouts of affected seed tubers that manage to emerge show curling of leaves, stunting and fading of leaves from green to yellow green.
  • The lower stems gradually rots away and the plant dies.
  • On tubers it exhibits small cream to tan water soaked surface spots that progress gradually inwards.
  • Decay expands rapidly resulting in rotting tissue that is slimy and water soaked.
  • This decay occurs very rapidly under moist conditions in storage leading to very unpleasant odors. The odors are due to secondary infections by other organisms.

Blackleg and soft rot on potato stem and tubers respectively. (


In storage, wet tubers having water/ moisture on the skin surface are easily infected.

  • Planting certified seeds from credible sources and Practicing crop rotation.
  • Control weeds like nightshades that harbor the disease organisms.
  • Avoid harvesting under wet conditions, dry wet tubers as quickly as possible.
  • Avoid bruising and do not wash tubers before storage.
  • Remove vines, clods and soil adhered to tubers before piling
  • Sanitize storage facility and eliminate condensation/ moisture during storage.
  • Keep well ventilated in storage.

Common Scab

It is most severe in soils with a pH greater than 5.2 or under drought conditions. The tubers are most susceptible, but stems and roots may occasionally be infected.

Common scab is caused by the bacterium streptomyces scabies which survives in the soil, on infected tubers and on vegetable debris. Infection occurs when the bacterium penetrates the thin tuber skin or enters through wounds and natural openings such as the lenticels (pores on tubers).

It can survive indefinitely in slightly alkaline soils but is rare in highly acidic soils. Transmission is through infected seed tubers, wind and water. It can also be transmitted through manure that is not well composted. (for composting and quality compost see Essentia Kanan).


  • No above-ground symptoms are apparent on infected potato plants and scab does not adversely affect potato tubers in storage.
  • Scab infections may appear as either raised or pitted lesions.
  • Raised lesions are dark, rough, corky areas on the tuber surface, while pitted lesions are sunken (1 to 3 mm in depth), dark-colored, corky areas.

Potato common scab (


  • Plant certified seeds and disease resistant varieties.
  • As much as possible rotate root crops by planting in alternate locations to limit diseases.
  • If the soil has a high pH, use acid type fertilizer. Do not use animal manure, wood ashes, or lime in the garden.
  • Keep potato plants well watered, especially during tuber set. However avoid excess watering.
  • Remove and destroy any potato debris and tubers at the end of the season. Try to plant potatoes in the same area of the garden only once every three to four years.

Fusarium Wilt

Fusarium wilt is a soil borne disease fungi (Fusarium oxysporum) that enters through the roots and interferes with the water conducting vessels of the plant. As the infection spreads up into the stems and leaves, it restricts water flow causing the foliage to wilt and turn yellow.


The symptoms of Fusarium wilt resemble those of Verticillium wilt. Laboratory analysis of diseased plant tissue usually is necessary to determine whether Fusarium or Verticillium is the causal agent. In most areas Verticillium wilt is more common than Fusarium wilt.

  • Lower leaves of infected plants turn yellow and wilt.
  • Leaf tissue between veins turns yellow then brown.
  • Wilting and yellowing of foliage progresses up the stems of affected plants.
  • Vascular tissue in stems and tubers often develops a brown discoloration.
  • Wilt symptoms are more severe when temperatures are high and plants are stressed for water.


  • Plant disease-free seed tubers.
  • Avoid growing seed potatoes in ground known to be infested with the Fusarium wilt pathogen.
  • Rotation out of potatoes or other solanaceous crops for 4 to 6 years is necessary to reduce soil inoculum levels.

References and Resources



Leave a Reply

Your email address will not be published. Required fields are marked *