Manitoba Weekly Vegetable Report

Weekly reports of insect pests and diseases observed by the Manitoba Vegetable Scouting Program

 | Carrots | Cole Crops |  Cucurbits |  Sweet Corn | Onions |  Others |

2008 Manitoba Vegetable Insect and Disease Situation Annual Summary

The Manitoba Weekly Vegetable Scouting Program continued in 2008, and focused on cabbage, cauliflower, cucumbers, sweet corn, onions, red beets, and carrots.

In many ways, 2008 was the opposite of 2007 when it comes to insect problems. The relatively low level of concern in 2008 can be attributed to severe weather that swept through the US Midwest early in our growing season. The heavy rains that largely wiped out much of the corn crop in that region also dramatically interrupted the migratory patterns of many of our pest insects. A substantial number of our more serious pests overwinter in the southern US and migrate over the course of the summer into the Manitoba region. These include pests such as diamondback moth, cabbage looper, and aster leafhopper. None of these were significant pests in 2008.

In general, disease problems were not of great concern in most vegetable crops. Red Beets, carrots, cabbage, and cauliflower were virtually disease free in 2008. Sweet corn provided the most interest in 2008, as Northern Corn leaf blight and Northern Corn Leaf spot, appeared early in the year, and created a great deal of concern among growers. At the conclusion of the season the concern with these two corn diseases diminished. Onion diseases were typical of past years, with soil-line rot and pink root observed in 2008. In storage however, neck rot was a serious problem that will lead to significant losses to the onion producers. Aster yellows in carrots in 2008, was observed at the lowest levels observed in the last ten years. Carrot leaf blights were also observed in 2008, but were of little concern.

TOP

Carrots

After the nightmare of 2007, 2008 was quite a relief from the tremendous populations of aster leafhoppers and the aster yellows problem. The differences between the two years were dramatic and informative. One of the main questions we are often asked is whether the aster leafhopper can effectively overwinter here in Manitoba. Given the incredibly high populations of leafhoppers in 2007, we were expecting, if leafhoppers do overwinter to any substantial degree, a fairly high spring population of leafhoppers. To that end, we were prepared and ready to sample early in carrots (which we do each year) but also in cereal fields nearby as cereals are preferred early season hosts and suitable as reproductive hosts for aster leafhoppers. We sampled five fields of wheat and five carrot fields throughout the growing season and were very surprised to find leafhoppers at incredibly low levels.

Aster yellows symptoms were detected for the first time on August 8th, 2008 at very low (trace) levels in one field. A week later, symptoms of aster yellows were observed in 3 of 5 carrot fields, at levels well below 1% (last year one field approached about 60% severity). At the end of the season aster yellows was observed in four of five scouted fields, again at trace levels (levels to low to make an accurate assessment of level), and there was likely no appreciable impact on yield.

If you look at the graphs below, the horizontal blue line represents the economic threshold. In 2007, at times, the population did not come below the economic threshold in spite of spray applications. In 2008, in both wheat and carrot fields, we did not exceed the economic threshold even once. Our highest aster yellows index value for 2008 was 42 and that was achieved in the last week of July. Our highest value in 2007 was approximately 4800. The economic threshold for resistant carrots carries an AYI value of 100. A comparison of pictures from 2007 and 2008 shows the dramatic difference in levels of infectivity as shown by symptoms in carrot fields.

Figure 1: Graph of the differences in the Aster Yellows Index values,
between 2007 and 2008 in carrots fields in Portage la Prairie Manitoba.

Figure 2: Graphs showing the differences in symptom incidence
between 2007 and 2008, in carrot fields in Portage la Prairie, Manitoba.

With respect to the overwintering of aster leafhoppers, we are fairly confident in suggesting that any leafhoppers that do overwinter make negligible contributions to the populations of leafhoppers that we see during the year. Most of the literature out there suggests that leafhoppers may overwinter in the egg stage in more northerly areas (Kansas, Nebraska) but do not contribute significantly to the following summer populations and this is what we observed. Winter conditions were not particularly harsh for overwintering, so there is no real reason for a precipitous crash in the population following the 2007 outbreak other than that the leafhoppers simply do not overwinter well in their northern range. From our perspective, they are largely a migratory pest that we need to watch for coming in on winds from the South.

Forked carrots (specific cause uncertain) were first observed July 11th in Portage la Prairie at levels of 12 and 20% in two fields. By the end of the season, forked carrots, were reported from all fields at levels of 8%, 12%, 20% 4% and 32%. It should be considered our sampling sizes within a week are rather small, so there is a great deal of variation in these numbers.

Cercospora (Cercospora carotae) leaf blight and to a lesser extent, Alternaria leaf blight (Alternaria dauci) were first observed on July 18th in four of the scouted carrot fields this year. By August 8th, 2008, Cercospora leaf blight was observed in all fields at incidence levels of 32%,16%,12%, 64%, and 52%. On the whole the damage due to the leaf blights was insignificant, as severity levels did remain low, throughout 2008.

TOP

Cole Crops

The only significant issue for cole crops this year in most fields was flea beetles. Populations weren’t tremendously high early in the season but as we see each year, once the canola has been harvested, feeding populations can be quite high in later cole crops. In general populations could be classified as moderate.

The caterpillar pests that have been significant in the past several years were thankfully absent for the most part. As mentioned in the general summary, cabbage looper and diamondback moth are both migratory and did not make it to Manitoba in any great numbers. We did find one, and only one, cabbage looper this year. It’s likely more were around, but most didn’t make it this far north. The imported cabbageworm (cabbage white butterfly) was not a significant pest as populations remained low following a couple of bad years back in 2005/06. Typically following a significant outbreak the natural enemies remain in sufficient numbers to keep populations low and this would explain the lack of problems with this pest.

Thrips did their usual bit after moving out of harvested cereal crops to create some fall havoc in the cabbage crops, but damage was well managed and not significant.

Cabbage diseases have been few and far between in 2008, sclerotinia head rot was observed on one plant, which is nothing unusual, and not a great concern in a field of several thousand plants.

Cucurbits

Unlike 2007, cucumber beetles, both striped and spotted, were largely absent from the fields and areas in which we carried out our scouting activities. We did see a single adult of the striped cucumber beetle, but that was it. Population patterns of the cucumber beetles are often quite variable from region to region, so while we did not see them in our scouting fields, they may have been present in other areas in the province. This is often the case based on phoned in reports that while we aren’t seeing anything locally, other regions may have much higher populations. This is a big reason why we always recommend scouting your own fields regularly because what we are seeing isn’t necessarily representative of what is found in your area.

On cucumber and other cucurbits scouted, symptoms of angular leaf spot (Pseudomonas syringae pv. lachrymans) were observed on July 18th on leaves in a planting near Portage la Prairie. Symptoms were not severe, and further spread of the pathogen appeared limited, with little economic damage reported.