J.F. Diamond ¹, A.V. Sturz ¹, P.W. Boswall ¹, T.L DeHaan ²and M.K. Keenan³

Introduction:

The family Potyviridae within the genus Potyvirus incorporates a large and economically important group of viruses. The common strain of the type species, potato virus Y^O (PVY^O), is considered one of the more destructive members, often causing severe yield reductions in potato crops. Other strains of PVY include PVN^N and PVY^C.
PVY^N may be detrimental to tobacco, but it is considered of little significance to the potato crop. The major concern with this strain in potatoes is the possible spread from potato to tobacco fields where these two crops are grown in close proximity and infection levels are high. PVY^C has not been found in North America. Since 1990, PVY^O has increased significantly in incidence in many areas of the U.S. and Canada and currently is considered a serious problem by many potato producers.
This document will summarize the current knowledge on PVY^O and detail practices that can be implemented to reduce the incidence of the virus.

In North America Potato Virus Y^O :

• can cause some of the most important virus diseases of potatoes, including severe mosaic and leaf drop streak.

• commonly occurs with PVX - causing rugose mosaic.

• reduces yields by 10 - 80 %, depending on potato variety and time of infection, and other factors.

Inoculum Sources:

Probably the two most important sources of this virus (from one season to the next) are infected seed potato tubers and infected volunteer potatoes.

PVY^Ois an obligate parasite and is unable to reproduce outside a suitable living host plant. Some weed species may be able to harbour PVY, however, little is known of their importance as hosts of the virus.

Transmission:

There are several aphid species known to have the ability to transmit PVY^O. By far the most efficient aphid vector has been shown to be Myzus persicae (Sulz.) - also known as the Green Peach Aphid. This aphid does not overwinter in P.E.I. due to our cold climate. Green Peach Aphids overwinter in the southern U.S.and move north to P.E.I. via wind currents. Data collected from aphid monitoring programs in P.E.I. indicate that the first inflights of Green Peach Aphids usually occur in early August.

However, at least 30 species of aphids, other than the Green Peach Aphid, can spread PVY^O. Aphids that do not colonize potatoes may still be responsible for some virus spread, as they move through potato fields "probing" for a more suitable host. Aphids feeding on infected plants can acquire PVY within a few seconds and they can transmit the virus immediately. Usually aphids retain the virus for less than one hour; however, retention for up to 24 hours has been reported.

Control Principles:

There are three general approaches to the control of PVY^O. These include:

1. Eliminating the source of the virus,
2. Preventing aphids from spreading the virus, and
3. Using resistant varieties.

1) Eliminating the Source of the Virus:

• Plant virus-free seed.

• Control weeds. While we are uncertain whether weeds are an important source of PVY^O, weeds such as wild rose, wild mustard, and wild radish are hosts of aphids on which large populations can develop.

2) Preventing Aphids from Spreading the Virus:

• The use of oil sprays may reduce the transmission of PVY^O from one plant to another by "washing" the stylet of probing aphids.

• Fast acting insecticides may be of use by rapidly reducing aphid populations thereby reducing within season spread.

3) Using Resistant Varieties:

• The use of resistant varieties is one method to eliminate significant yield losses to PVY^O.(Table 1).
  
  
• Breeding programs are aimed at producing immune or resistant varieties or plants that exhibit distinct symptoms when infected so plants can be rogued easily.
  
  
• The current concerns regarding PVY^O in both the U.S. and Canada is thought to be compounded by the increasing acreages grown to the varieties, Shepody and Russet Norkotah. Under certain conditions - environment (e.g. temperature, fertilization rate) - these varieties do not show distinct symptoms and so infected plants cannot be rogued effectively. These varieties can then become a source of infection which can be spread by aphids to other potato fields.

Control Summation:

• Use insecticides to prevent the population of aphids from increasing within a field. Follow label recommendations for application.

• Proper use of oil sprays may reduce the spread of PVY^O. At the time of writing registration of oil sprays was still pending.

• Plant seed certified through a recognized seed potato inspection program. Field readings and post-harvest test results may be used as guides to select seed lots with the lowest virus levels.

• Rogue as early as possible.

• Avoid planting seed potatoes downwind from commercial fields.

• Control weeds and volunteer potato plants.

• Topkill seed fields early to prevent late-season virus infection.

• Avoid planting susceptible varieties in close proximity to fields with varieties which have poor symptom expression.

An asterisk (*) under the disease resistance heading means the variety is considered neither resistant nor particularly susceptible.

For further information on variety resistance, refer to Publication 1300A "PotatoCrop - Variety, Weed, and Pest Control Recommendations".

The authors wish to recognize Drs. Guy Gooding, Peter Ellis, and Rudra Singh for their help in preparing this document.

References:

1. P.E.I. Department of Agriculture, Fisheries & Forestry, P.O. Box 1600, Charlottetown, PEI, C1A 7N3.
   
   
2. Agriculture and Agri-Food Canada, Food Production & Inspection Branch, Seed Certification Laboratory, P.O. Box 1240, Charlottetown, PEI, C1A 7M8.
   
   
3. P.E.I. Potato Board, 420 University Ave., Charlottetown, PEI, C1A 7Z5.
   
   

4. For further information contact A.V.Sturz at the Prince Edward Island Department of Agriculture, Fisheries and Forestry.