DISEASES AND THEIR CONTROL
Potato Leafroll Virus (PLRV). This is a disease which significantly reduces yield by 30-50% depending on the variety and strain of the virus.
Leafroll virus is transmitted by green aphids (Myzus persicae). However, other species of aphids can also serve as vectors.
Leafrolling is the main symptom of the disease. The edge of mature leaves rolls upward becomes dry and thickens. The leaves easily break when pressed. The plants grow upright become stunted and are light green. In some potato varieties the underside or base of the leaves turn reddish or purplish.
To control potato leafroll virus: 1) plant certified seed potatoes, 2) remove and destroy diseased plants, 3) control aphids, 4) eliminate volunteer plants, 5) isolate seed potato area from fields intended for table potato production.
This disease is due to the presence of different viruses singly or in combination (PVY, PVY + PVX, PVY + PVX + PVA). The disease affects the potato seed stock.
PVY is mainly transmitted by aphids. Symptoms of the disease are: yellowing of the leaflets; leaf drop; premature death of plants; dwarfing of plants and mottling and crinkling of leaves.
PVX is the most widespread potato viruses and causes yield reduction of up to 15%. It is often field transmitted through the sap in contact with transfer agents like wind, animals, machinery, other plant roots, cutting knives, infected sprouts and biting insects. Symptoms are: 1) dwarfing of the plant and reduced leaflet size; 2) in combination with A or Y, X causes crinkling, rugosity or necrosis.
To control rugose mosaic: 1) plant disease free or certified seed tubers 2) remove and destroy diseased plants, 3) control insect vectors (aphids), 4) remove all volunteer plants, 5) observe strict sanitation when cutting seedpieces.
Late Blight (Phytophthora infestans (Mont.) de Bary). It is the most destructive disease of potato. At present, it remains a limiting factor in potato production in the Philippines.
It disease affects leaves, stems, and tubers. It appears on the leaves as pale-green, irregular spots. In moist weather, spot widen heir centers and turn dark brown or black. On the lower sides of the leaves a whitish ring forms around the dead areas.
Stems and petioles turn brown when infected. Under damp conditions the whole stem may turn black in a short time and die.
Tubers get infected in the soil by rain-borne spore from blighted tops and at harvest by contact with blighted stems or leaves. The first symptom of the disease in potato tubers is the presence of a shallow, reddish brown dry rot that spreads irregularly from the surface through the flesh. Soft rots often follow the late blight rot and can completely destroy the tuber.
To control late blight:
- plant certified seed-potatoes
- destroy or eliminate all potato cull piles
- plant resistant varieties such as Granola, Conchita, and Greta during rainy season
- spray with any of the fungicides like Dithane M-45, Manzate 200, Manzineb, Curzate-M, Daconil, or Defolatan/4F.
Rhizoctonia canker (Rhizoctonia solani Kuhn)
The causal organism can be transmitted through tubers. It can thrive on several hosts and survive a wide range of temperatures. It is destructive to emerging sprouts particularly when the soil is cold and wet.
Long, brown lesions occur on affected shoots, stems and stolons. The sprouts often decay before reaching the surface of the soil. Roots also decay if infected. This usually results in miss or gaps in the plot. When plants are attacked after they emerge, affected stems become covered with mycelia.
Infected plants are characterized by a general lack of vigor, foliage curling and turning pinkish or purplish. Often, small green or reddish tubers above the soil surface are formed as a result of the interference of starch translocation.
The tubers which are small and numerous form into a tight cluster, thereby forming an abnormal appearance. In mature tubers, the most common symptom of the disease is the presence of numerous, hard, dark spots on its surface.
Control the disease by:
- using certified seed potatoes,
- planting chitted or pre-sprouted tubers,
- avoiding cold and wet soil for planting,
- covering the seed pieces with not more than 5 cm of soil to promote rapid growth of the plants
- practicing crop rotation with cereals.
Common Scab (Streptomyces scabies (Thextes) Walksman and Henric). Common scab is a soil-borne disease which occurs throughout the world. It is most prevalent in neutral and slightly alkaline and light sandy soils, especially during dry season.
Symptoms are observed mostly on tubers where small, brownish, and slightly raised spots appear. These spots enlarge, coalesce and become corkly on the surface. The disease makes the tubers less edible an unmarketable.
To prevent the establishment of the disease in scab-free field:
- use scab-free seed potatoes for planting
- practice 3-5 year crop rotation, preferably with legumes
- do not apply large amounts of fresh manure in the field
- maintain high soil moisture levels during and after tuber set for 4-9 weeks
- avoid overliming.
When growing potatoes on mildly infested soils, use sulfur and acid fertilizers like amonium sulfate or urea.
Blackleg (Erwinia corotovora ssp. corotovora Dye). The disease affects the seeds, stems and tubers.
At the early stage of infection, leaves turn yellow and roll even when the plants have just emerged from the soil. As the disease progresses, the stem in contact with the soil becomes black including the stolons and tubers. Moisture and high temperature hasten the progress of these symptoms which leads to plant wilting and death.
Control measures include:
- planting of certified seeds
- avoid cutting seed tubers
- practice crop rotation with cereals, legumes and other non-host crops
- practice field sanitation
- destroy potato cull piles which serve as source of disease
- remove infected potato plants and tubers to reduce the amount of infection in the field or storage.
Bacterial wilt (Pseudomonas solanacearum E.F. Smith)
A bacterial disease of potato that is extremely destructive in tropical and sub-tropical regions. The causal organism lives in the soil and is distributed through irrigation water and farm floods. Root to root contact also transmits the bacterium.
The first symptom of the disease is slight wilting of the plant during the hottest part of the day. Affected plants recover during cool nights but the wilting continues until the plants die. The center of the stem is brown when cut. Grayish white droplets of bacterial slime ooze from the cut stem when suspended in a glass of water.
If an infected tuber is cut crosswise, a brownish circle appears underneath the skin and sticky and milky slime comes out.
Control measures include: 1) planting certified seeds in non-infected soil, 2) rotating potato with non-solanaceous plants like cereals, legumes, etc., and 3) eliminating volunteers and solanaceous weeds which serve as alternate hosts of the disease.
Plant parasitic nematodes are tiny and earthwormlike. They feed by piercing and sucking the plant juice. They damage roots, stems, buds or leaves. The root-feeding types are the most common in Benguet and Mountain Province.
A number of plant parasitic nematodes have been noted in potato growing areas. The most destructive is the potato golden nematode or cyst nematodes (Globodera rostochiensis Woll) This pest can cause losses of up to 15% on potato crops without visible symptoms. Complete economic crop failure can result when nematode densities are high.
The symptoms are wilting, stunting, and premature aging of the plant. Potato affected by this pest may resemble plants having root injury due to drought. Infected plants display symptoms of nutrient deficiency such as chlorosis, stunting, wilting during hot periods of the day, and early death. Plant growth is generally poor. The roots of affected plants are shallow, short and may show rotting, swelling or galls of different sizes.
Nematodes are widespread. But some areas like forest sites which have been recently opened for farming, may still be cleared of nematodes. In such instances, quarantine measures must strictly observed. Seedpieces to be used in these sites should also be carefully inspected.
One practical means of bringing down the pest’ population level in infested areas is by incorporating chicken manure into the soil before planting.
Rotation of non-host crops can also decrease the incidence of nematodes, but nematode-susceptible crops such as tomatoes, eggplants, cucurbits, carrots an beans should not be grown.
Nematodes unprotected by living roots die early when exposed to sunlight. In nematode-infested fields, pull out the roots of the harvested crop and other residues.
The use of resistant varieties like Granola is also one way of controlling nematodes.
Potato cyst nematode can also be effectively controlled by a biocontrol agent, Paecilomyces lilacinus. Simply dip the potato tubers in the fungal suspension for 10 minutes before planting.
Part 1 | Part 2 | Part 3
source: Baguio National Crop Research Development Center, Guisad Valley, Baguio City