Aster Yellows and Witches’ Broom

These virus-like diseases (phytoplasmas) are spread by leafhoppers from nearby weeds or forage crops. Aster yellows causes the upper leaves to roll and become yellow to purple in colour. Tubers are soft and small. If planted, such tubers produce only very weak plants. Witches’ broom causes dwarfing of plants, a proliferation of stems, small leaves and numerous tiny tubers. If infected tubers are planted, a mass of shoots will sprout forming a low mound of foliage.

Control
Isolate plantings from forage legume fields where leafhoppers may be numerous. Control leafhoppers in and around potato fields (see “Leafhoppers”, this section).

Bacterial Ring Rot (Clavibacter)

This disease is characterized by a cream-yellow to brown decay in the vascular ring of the tuber. External symptoms such as discolouration just under the periderm and skin cracks may also be present. In the later stages of disease the entire centre of the tuber breaks down leaving only a thin, outer shell. Tuber symptoms may easily be confused with other disorders. Plants in the field may remain symptomless, or show varying degrees of wilt. Wilted leaflets develop interveinal yellow to necrotic areas and tend to curl upward at the edges. The bacteria spread from infected to healthy seed by way of contaminated equipment and storage containers.

Control
Plant only B.C. certified seed potatoes if possible. Use whole seed if practical. Practice crop rotation and good general sanitation. Avoid using second hand sacks especially for seed. Thoroughly clean and disinfect custom or shared seed cutters and planters between farms. Effective disinfectants are:

If bacterial ring rot is suspected, contact your local BCAGRI office or the BCAGRI Plant Diagnostic Laboratory at Abbotsford (604 556-3126).

Bacterial Soft Rot (Erwinia, Clostridium)

Several different species of bacteria may be involved in soft rot but all can overwinter on tubers and may be present in soil and especially in water used for irrigation. Wet, cream-coloured decay, often developing a foul odour in advanced stages, can develop in seed pieces after planting, in tubers prior to harvest, or in harvested tubers. Tuber symptoms are most likely to occur with the early and mid-season crop when tubers are harvested under wet and/or warm conditions. Blackleg symptoms may develop on injured stems or under hot field conditions.

Control

1. Avoid excessive soil moisture.
2. If possible, harvest when soil temperatures are less than 10°C. Minimize mechanical damage to tubers during harvesting and handling.
3. Protect harvested tubers from hot sun and desiccation.
4. Cooling potatoes after harvest during hot weather will reduce losses.
5. If it is necessary to wash tubers before marketing, dry them as soon as possible and package them in well-aerated containers.

StorOx (formerly called Oxidate) is registered for control of Bacterial Soft Rot in storage. Follow label precautions carefully to avoid operator and environmental hazards (see Storage section, this chapter, for more details).

Black Scurf & Rhizoctonia Canker

This widespread soil fungus causes red to dark-brown cankers on young sprouts and later infections cause brown lesions on stolons and roots. A distinctive symptom is the formation of small aerial tubers on infected stems accompanied by a curling of the upper leaves. Infected plants do not die down normally. Potato tubers have small black specks (scurf) on the surface.

Control

1. Rotate with sod and cereal crops, but be aware of the danger of wireworms which build up in those crops.
2. Avoid planting in cold wet soil.
3. Use seed free of black scurf, if possible.
4. Harvest promptly once the skins are set to reduce the amount of black scurf on the tubers.
5. Treat seed pieces with Maxim PSP (controls seed borne Rhizoctonia only). See Table 4, Potato Seed Piece Treatments.

Blackleg (Erwinia)

The soft rot bacteria causing blackleg overwinter in infected volunteers and in the lenticels of symptomless tubers. Dark brown or black discolouration occurs at the base of the stem and tubers may show black rot at stem end. Individual stems or entire plants wilt. The disease is favoured by cool, wet soil, continuous potato cropping and infection by tuber rot fungi such as Fusarium.

Control

1. Disinfect the cutting knife after cutting partially rotted tubers.
2. Disinfect custom or shared seed cutters or planters between farms (see Bacterial Ring Rot, this section).
3. While seed-piece dust treatments do not directly control the black leg bacterium, they prevent early rotting of the seed-piece and have been found to indirectly reduce field incidence of black leg (see Fusarium Rot). See Table 4, Potato Seed Piece Treatments.
4. Use of small, whole seed is an effective preventive measure when practical.
5. Remove cull piles, refuse and volunteer potatoes.

Early Blight (Alternaria solani)

Early blight occurs in all areas of the province but is more common in the Interior than it is at the Coast. Early blight first appears as small black spots on leaves which may enlarge to one cm or more in diameter. Infected leaves eventually become yellow and necrotic. A severely blighted plant will have hundreds of infections on it, in contrast to a plant infected with late blight which usually has only a few extensive infections which expand continuously until all the foliage is rotted.

Control

1. If possible, turn under crop residue promptly after harvest.
2. Avoid replanting fields which had a severe early blight problem the previous year.
3. Ensure adequate nitrogen fertility.
4. Keep weeds under control.
5. A spray program for late blight will also help to control early blight (see Table 3, Potato Disease Control).

Fusarium Rot

Dry Rot and Seed-Piece Decay — Decay occurs in storage or after seed-piece is planted.

Control

1. Wait until soil warms up before planting.
2. Seed piece treatments - originally intended for protection against fusarium dry rot, seed piece treatments now have a bigger role to play. It is recommended that a seed piece treatment also contain some mancozeb to provide protection against the transfer of late blight spores at seed cutting time. The following products all contain mancozeb.
3. Products containing captan or thiophanate-methyl (Senator PSPT), or fludioxonil (Maxim PSP), or metiram (Polyram 16D) will control dry rot, but not late blight spores. See Table 4.
4. To reduce losses in storage, especially in crops which are to be stored for a long period or in processing crops which must be stored at warm temperatures, a post-harvest fungicide application may be beneficial.  Apply Mertect in a fine spray to all surfaces of tubers. Use 7.5 L of Mertect per 170 L of water applied at 2 L of this solution per tonne of potatoes. Where resistant strains of Fusarium and/or silver scurf exist, the Mertect treatment will be ineffective against them.
5. Another option for post-harvest control of table and processing potatoes is Stadium (azoxystrobin/ difenoconazole/ fludioxonil).  Apply Stadium using the rate of 32.4 mL per tonne of potatoes at a final spray solution (total slurry volume) of 2 L per metric tonne of potatoes.  Treated potatoes should be barely wet.  Apply as an in-line aqueous spray ensuring that tubers are rotating along a conveyor into storage during treatment.  Also, ensure tubers are tumbling as they are treated with a full flow of potatoes maintained on the conveyor (reducing open space between tubers).  Do not apply to seed potatoes.  Do not make more than 1 post-harvest application to the tubers.  Also provides suppression of Siver Scurf (see Silver Scurf section).
6. For seed potatoes only, a more reliable treatment for Fusarium dry rot is Dithane F-45. Apply 1.58 L of Dithane F-45 in 4 to 8 L water per tonne of seed potatoes as a conveyer belt spray going into storage. Treated seed must not be used for food or feed.

StorOx (formerly called Oxidate) is registered for control of Fusarium Rot in storage. Follow label precautions carefully to avoid operator and environmental hazards (see Storage section, this chapter, for more details).

Late Blight (Phytophthora infestans)

Late blight occurs in all areas of the province but it is more dependent on rainfall than early blight and is more likely to reach epidemic proportions at the Coast. Water-soaked to gray spots appear at tips or margins of leaves, in leaf axils and on stems. Spots spread over plant surface. Rate of spread increases in moist weather and under sprinkler irrigation. Blight causes dry or wet rot of tubers in field and in storage.

Monitoring
Potato integrated pest management (IPM) in the Fraser Valley involves commercially available insect and disease monitoring services. Monitoring for late blight provides growers with the information needed to accurately time pesticide sprays and avoid unnecessary spraying.

Control

1. Avoid replanting fields which had severe late blight the previous year.
2. Destroy culls and volunteers. Remove them from the farm or cover completely with a plastic sheet so that the fungus will not grow and sporulate. Kill any sprouts that appear on cull piles with herbicides.
3. Carefully grade out suspicious looking tubers when preparing seed for planting. Discard tubers that are partially decayed or mummified but still sound enough to send up some sprouts. Dispose of them in a secure manner off your farm if possible and where they will not be able to grow. The seed to be planted should be treated with a fungicide containing mancozeb, which has been shown to reduce late blight transmission from infected tubers.
4. If using floating mulches or row covers on early potatoes, apply a fungicide as soon as the cover is removed. Curzate 60 DF is the preferred material as it has some “kick-back” activity on blight infections. Covers encourage early succulent growth and blight infections. Blight becomes established in these earlier and then spreads to later crops.
5. Separate fields of early, second early and late crops as much as possible to slow down the spread of spores from the earlier to the later crops. Early crops should not be planted upwind of later crops.
6. Ensure potatoes are well hilled.
7.  Begin a spray program as soon as advised by your IPM scout or before row closure. When disease pressure is severe the following spray program is suggested:
   • Apply a systemic fungicide such as Curzate 60 DF, Acrobat 50WP, Quadris F, or Tanos at least once before row closure. Each of these fungicides is permitted a maximum of 3 applications per year.
   • Apply products listed in the table below through the growing season as recommended by your IPM scout or on a 5 – 10 day schedule.
   • Apply Acrobat 50WP to cover soil and stems after top-kill if blight is present or conditions are wet. See also Potato Blight Fungicide Table below.
8. If blight gets out of control, top-kill the crop and apply one more Acrobat 50WP application when the tops are about half dead. Note: The PHI for Acrobat is 4 days.
9. Harvest in dry weather when vines have been dead for at least two weeks. This will reduce chances of tuber infection during the harvest operation and will also allow recognition and grading out of tubers that were infected in the field. If infected tubers and wet soil go into storage, the pathogen may sporulate and spread to other tubers.
10. Solanaceous weeds such as Hairy nightshade are hosts of late blight. Check adjacent crops and weedy areas around the edges of fields for this weed and if found, it should be controlled to prevent a build-up of inoculum.

Leaf Roll

Leaf roll is caused by a virus which overwinters in the tubers and is carried from diseased to healthy plants by green peach aphids. If diseased tubers are planted, lower leaves appear rolled and stiff. Symptoms appear when the plant is about 15 cm high. If infection occurs in the field (current season infection), leaf roll symptoms rarely appear on the lower leaves and may or may not appear on the upper leaves. Current season infection causes net necrosis in the tubers of Russet Burbank and a few other varieties. Tuber necrosis may increase in storage.

Control

1. Use only certified seed.
2. Rogue seed fields early and often, including all volunteers (carry plant and seed-piece clear of field and bury if practical).
3. Do not plant susceptible varieties such as Russet Burbank in fields which have volunteers from the previous year or adjacent to fields with volunteers.
4. Winter flooding can be used to kill tubers in the field and prevent volunteers in areas with adequate natural flooding and in dyked fields with water control.
5. If you are growing your own seed, set seed plot away from edge of field.
6. Control aphids. Early and mid-season varieties should be sprayed, not later than mid-July, to prevent aphid build-up and migration to late potatoes, where they may cause net necrosis in Russet Burbank and some other varieties. Spray all seed potatoes for aphids after mid-July.
7. Top-kill seed crops early to avoid the fall migration of winged aphids.

Leak (Pythium Rot) and Pink Rot (Phytopthora)

The organisms causing leak and pink rot are closely related to late blight but are soil-borne and attack the roots and tubers. Pink rot infects tubers in the soil prior to harvest while leak infection occurs during harvest and handling, especially during warm weather. Affected tubers may appear normal on the outside, but feel soft. If squeezed, liquid oozes out. Internally, the tuber is broken down and dirty-white to gray in colour with a black ring in the case of leak. Tubers infected with pink rot will turn pink after they are cut and exposed to the air.

Control
Land-leveling and drainage to eliminate wet spots and rotation away from fields with a history of pink rot or leak are recommended. Avoid heavy irrigation on dry, hot soil. Ensure tubers have a good skin set before digging. Do not leave tubers on ground in hot sun. During hot weather, dig between midnight and 10 a.m. Pre-cool to 13°C, and specify transit temperature of 13 to 16°C. Prevent injury to tissue. Keep tubers dry and cool during harvest and storage.

If planting into a field with a history of either disease, consult Table 3, Potato Disease Control.

Mosaic Virus (PVY)

Mosaic is caused by one or more of potato viruses X, S, Y, M and A. Potato virus Y exists in different strains. PVY (o) occurs in Western Canada. PVY (n), which does not cause symptoms on potato but is damaging to tobacco, occurs in other parts of the world. Leaves of mosaic-infected plants are mottled, may be wrinkled and distorted, or have veinal necrosis. Plants are stunted and yields are reduced.

Control
Use certified seed. Rogue early and often as for leaf roll. Control aphids.

Root-knot Nematode

The northern root-knot nematode is a minor pest of potato in some areas of British Columbia. The Columbia root-knot nematode, newly discovered in the United States and established in the Lynden area in Washington State, is apparently not present in Canada. It is a serious pest of potatoes in the Columbia Basin and other areas of the Pacific Northwest. The Columbia root-knot nematode can reproduce faster and at lower temperatures than the northern root-knot nematode, and is considered a potentially serious threat to the B.C. potato industry. Quarantine regulations are in effect to avoid entry of infected seed potatoes and other plant products, as this nematode is spread mainly in seed potatoes and infested soil.

Small protuberances can be detected on roots and tubers of infected plants. High populations cause wilting and, premature senescence of foliage, and disfiguration of tubers. Nematode analysis of soil and root samples is necessary for the diagnosis. If root-knot is suspected, submit a root and/or tuber sample to the BCAGRI Plant Diagnostic Lab at Abbotsford. To have a soil sample tested, contact a nematode testing service to determine when and how to collect the sample.

Control

1. Use seed potatoes certified free of root-knot nematode.
2. Destroy all culls and volunteers from previous potato crops.
3. Where the northern root-knot nematode is present, rotation with small grains, grasses and corn is recommended, while alfalfa should be avoided.
4. Fumigate in late summer with Vapam according to label directions.

Scab, Common (Steptomyces)

The organism is present in the soil or on the seed and causes round or oval, raised, corky spots or thick, irregular roughening/russetting on the surface of tubers.

Control
Practise long rotations with small grains, grasses and corn. Plow under green cover crops. Apply acid-producing fertilizers and limited quantities of sulfur, 225 to 325 kg/ha (90 to 130 kg/acre). Avoid alkaline-producing soil amendments like lime, ashes or nitrate of soda. Manure, especially poultry manure, should be applied in the fall, not in the spring just prior to planting. Maintain good soil moisture at time of tuber formation. Plant clean seed.

Silver Scurf (Helminthosporium)

Silver scurf is a fungal disease transmitted largely by infected seed pieces. Soil transmission may occur to a limited extent. Potato is the only known host. Infection takes place through the lenticels and skin before tubers are dug. Only skin tissue is affected. Extensive lesion enlargement and some new infections may occur in storages in which temperatures exceed 3°C and relative humidity is at least 90%. Symptoms consist of silvery blotches on the skin and there may be considerable loss of colour on red varieties.

Control

1. Avoid planting infected seed pieces.
2. Avoid planting potatoes in fields where volunteers are likely to grow.
3. Harvest tubers as soon as they are mature.
4. If there are no resistant strains of the fungus present, seed-piece treatments (eg. Maxim PSP and Senator PSPT) and the Mertect treatment recommended under Fusarium Rot (this section) may be of some value. These treatments will be of no value if resistant strains are present. See Table 4 (Potato Seedpiece Treatments) for more information.
5. For suppression in table and processing potatoes, use Stadium (azoxystrobin/ difnoconazole/ fludioxonil) as a post-harvest treatment.  See Fusarium section for rate and precautions.  Do not use on seed potatoes.  Do not make more than 1 post-harvest application to the tubers.

StorOx (formerly called Oxidate) is registered for control of silver scurf in storage. Follow label precautions carefully to avoid operator and environmental hazards (see Storage section, this chapter, for more details).

Verticillium Wilt (Early Dying)

The fungus survives in the soil or infected plant parts. It invades through the roots causing premature yellowing and death of the vine. Yields can be greatly reduced when disease is severe. It is more prevalent in the Interior.

Control

1. Practice crop rotation, avoiding tomatoes, peppers, eggplants and melons.
2. High rates of nitrogen fertilizer delay the appearance of symptoms.
3. Soil fumigation is used in Washington but is usually not warranted nor would it be very effective in coastal B.C. It may be necessary for continued potato production on some of the lighter soils in the B.C. Interior.
4. Windrowing and burning infected tops promptly after harvest will reduce inoculum carryover but may encourage scab in succeeding potato crops.
5. Senator seed piece treatment is registered for verticillum wilt. See Table 4, Potato Seed Piece Treatments.