PROJECT RESULTS

Developing Control Strategies for Aster Yellow Phytoplasma Associated with Carrots in Manitoba

Dr. Fouad Daayf

Department of Plant Science

University of Manitoba

Winnipeg, Manitoba  R3T 2N2  Canada

Table of Contents:

• Background and Objectives
• Procedure and Project Activities
• Results and Discussion

ARDI Project:

#00-464

Project Status:

Completed April, 2003

Background and Objectives:

Aster yellows describe a disease caused by phytoplasma, carried by leafhoppers and transmitted to plants as these insects feed.  In the last few years, this disease severely affected carrot crops in Manitoba.  Due to the complex nature of the insect- phytoplasma- plant-­environment interactions, aster yellows disease management and control in carrots require a multidisciplinary approach, including a fast and accurate detection system.

The main objectives of this project included:

1. The use of molecular techniques, namely PCR, for a more accurate determination of aster yellows incidence in selected carrot fields in Manitoba.

2. Using such technology on species other than carrots for possible identification of alternate sources of inoculum.

3. Using molecular techniques, namely RFLPs, for characterization of phytoplasmas isolated from both carrots and leafhopper vectors.

4. Using such knowledge towards investigating leafhopper population dynamics in the field.

5. Suggesting additional control strategies to combat aster yellows phytoplasma in carrots, based on the generated knowledge.

Procedure and Project Activities:

Five carrot fields in the Portage la Prairie area were selected to determine aster yellows disease incidence in Manitoba in both 2001 and 2002.  During each visit, fields were scouted for aster yellows symptoms on carrot and other plant species, and both healthy plants and those suspected to be infected with aster yellows were collected for molecular detection of the pathogen.  At the same time, insect populations were monitored (over 1,000 and 1,500 leafhoppers for the five fields in 2001 and 2002, respectively) and molecular techniques were used to detect and characterize the aster yellows phytoplasma they harbour.

Results and Discussion:

In this project, many accomplishments could be enumerated:

• Accuracy of visual symptom determination was estimated for both healthy- and diseased-looking samples.

• We were able to adapt PCR techniques to our carrot plants and to leafhoppers, and phytoplasma detection was successful in both types of tissues using slightly different protocols.

• Phytoplasma found in Manitoba in this study were classified either in sub-group I-A or I-B, same as those found in Alberta.  Multiple infection with phytoplasma from both sub-groups were found in both carrots and leafhoppers.

• Aster yellows phytoplasmas were found in other plant species, which can represent a source of inoculum.

• The risk of aster yellows phytoplasma is higher during the early season, rather than middle and late season, based on the percent infection of leafhoppers.

• Given that risk of infection is higher early in the season, protective measures should be applied both early and later in the season.  Protective measures should be applied and decisions should be made on the basis of presence of leafhoppers during the early part of the growing season (May, June).  This is contrary to current management practices relying exclusively on the Aster Yellows Index (AYI) as a decision making tool.

• The use of AYI might be subject to some error given that it is derived from populations of leafhoppers from Wisconsin.  The subsequent leafhoppers that arrive in Manitoba have had ample time to pick up more inoculum, hence may have a higher rate of infectivity.  This is confirmed by this study in that few leafhoppers are present early in the season yet the infectivity rate is much higher than typical AYI estimates.

• Later season populations should be monitored and the AYI can remain as a decision making tool.  The reason it remains effective is that Manitoba populations of leafhoppers are less infective as a whole, later in the season, and the AYI estimate may more closely correlate with Wisconsin estimates.  This has the added benefit or reducing or limiting pesticide use later in the season.

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