Major diseases that affect the potato include late blight (LB), bacterial wilt (BW), and viruses. These diseases can lead to yield losses of 20–80%. Root rot and early blight do occur, but on a relatively smaller scale. Controlling LB and BW has always been a challenge to many farmers. Infected seed plays an important role in disseminating diseases, both locally and over considerable distances. It is not the heavily infected tuber that is the problem since these generally rot away, only maintaining contamination of the land in which they were grown. However, slightly infected tubers, which show no visible symptoms, pose a serious threat of spreading the disease to new areas. Self-sown potatoes are extremely difficult to eradicate and, if a paddock is infected, the disease may remain in it for five or six years after the initial outbreak. Bacteria can also be spread to clean tubers from an infected seed-cutter. There is also a real danger of infection if secondhand bags are used or if half ton bins have held infected potatoes. Growers should be aware of these risks and take precautionary measures.

Bacterial wilt is difficult to control (or eradicate) because of the soil-borne nature of its causal organism. However, some options are available managing the disease. These options include crop rotation with pastures, cereals, and non-solanaceous crops for periods exceeding five years, use of certified seed from reliable sources, and regular crop inspection for disease symptoms and remove and destroy diseased plants, tubers, and immediate neighboring plants. Spraying with appropriate measures can reduce the incidence of the disease on farms. Farmers mostly spray their farms with fungicides as counter measures against these diseases. The application is conducted about three times a season at a low rate of 1.24l per hectare. Other measures include uprooting of diseased plants and crop rotation, albeit on minimal scale. Control of pests is virtually absent in most potato farms, even though pests (mainly leave miner, white fly, spider mites and aphids) are seen on farms. The crop protection measures adopted by farmers are largely ineffective, as the diseases continue to devastate farms.

An eight week spraying program with weekly sprayings was developed to control late blight and insect pests. This spraying program proved to be an effective approach to control late blight, bacterial wilt, and insect pests among potatoes. Furthermore, it is of importance of to identify the pests and diseases in order to apply appropriate measures that discourage the development of pests and keep pesticides and other interventions to levels that are economically justified and reduce or minimize risks to human health and the environment. In Kenya, the LB-severity assessment was conducted bi-weekly using a visual disease assessment scale on the demonstration plots.

Comparison was made on the disease development between the four options of crop protection (Spray program 1 (SP1), Spray program 2 (SP2), farmers’ practice (FP), and research recommendation (research rec)) that were being demonstrated. The farmers’ practice included 3 sprays of Ridomil/Mancozeb during 8 weeks. The research recommended practice consisted of 4 sprays of Ridomil/Mancozeb during 8 weeks. From a research perspective the first spraying is very critical to the potato crop and two weeks after emergence is an ideal time to do this.This is mainly to protect the crop at this tender age when it is very vulnerable to the disease, as it will be very difficult for a crop to compensate for the yield loss from an early disease infection.

The spraying programs 1 and 2 included weekly and sprayings for 8 weeks of the following active ingredients: The sprayings of SP1 and SP2 started two weeks after the emergence of diseases as suggested. If the disease pressure is high, however, then the application frequency can be increased to twice a week.