 |
|
| | Archive |
|
| September 22, 2006 Issue Number: 15 |
| Weekly Report: |
This fall, the same folks who brought you the Weekly Vegetable Report, will begin work on the 2007-2008 Edition of the Guide to Vegetable Crop Protection. It will feature all the crop protection products (insecticides, herbicides, and fungicides/bactericides) that are registered for use on vegetables in Manitoba (and most other provinces in Canada). Information is displayed both as abbreviated labels, and in look up tables to provide information quickly for each vegetable. Beneficial cultural practices that assist in disease and insect management will also be included. It should be available for purchase in Mid-March 2007. Please check and bookmark The Manitoba Vegetable News website over the winter to learn when it will be available for order.
For the 2007 season, we hope to make the report more easily accessible for those of you using dial-up and having difficulty downloading the report due to the number of pictures.
We also welcome any suggestions, comments, hate mail, etc. that could improve the report. We can by no means guarantee that all suggestions can be incorporated, but all suggestions are taken seriously, and will be considered for the 2007 report. Our goal is to develop a report that can inform, educate, and improve insect and disease management in Manitoba (and beyond?).
2006 Manitoba Vegetable Insect and Disease Situation in Review
In 2006, the first weekly report was released on June 16th, 2006 and continued until the 14th report on September 15th, 2006. All these and reports and those from 2005, and 2004 can be viewed using the archive link, located in the top right hand corner of the report field.
In 2006, the theme was irrigation, irrigation, and more irrigation, as this turned out to be a year with very little precipitation and temperatures in the high 20’s and mid 30’s.
Onions:
Overall, the onions this year appeared to be in good shape. Development appeared to be more advanced relative to 2005, and harvesting operations began earlier than usual. The graph below shows development of a number of varieties in Manitoba (each symbol represents a different variety). Time constraints this year didn’t allow this work to be as complete as initially intended, but over several seasons, the data gathered will assist in determining what impact the events of a season have on crop development.
The first onion disease encountered this year was onion smut (in the June 30th, report), caused by the fungus Urocystis cepulae  .This disease is soilborne, and the fungus can persist in soil for a number of years, indicating the necessity of crop rotations, to minimize the impact of this fungus. Generally, smut can be a problem on plants that have been under stress, plants that are kept healthy rarely have a problem. Seed treatments do offer added benefit to managing this problem. The black marks seen on the picture below represent masses of thousands upon thousands of spores.
Fusarium basal plate rot (Fusarium oxysporum f.sp cepae) generally shows up every year in Manitoba onion fields, and 2006 was no different. The fungus is soilborne and can survive for years in soil. Generally this problem is considered, a higher temperature disease, with optimum conditions for the outbreak of disease ranging from 25-28°C. Rotations of 4 years or longer away from Allium crops (onions, garlic, shallots, leek) are recommended, especially if high levels are found in a season. There are varieties with resistance to infection. Any stage of onion growth can be affected. Rots can progress in storage, though usually the symptoms are quite noticeable, that they can be graded out prior to storage.
Of all the threats to onion production, the disease arising from Botrytis fungi are the most common, the most widespread, and potentially the most damaging to the industry. Relative to the 2004 and 2005 seasons, it will be cautiously stated that the 2006 season was much more of a relief. The fungi did not go away and were still quite active, as Botrytis cinerea (soil line rot, brown/coffee stain) and Botrytis allii (neck rot) were detected in 2006. While the growing season has ended, the storage story still has to be told, as both of these fungi can continue to cause rots in storage. Botrytis allii, has few external symptoms, apart from a softening of tissues at the neck (and sometimes at the base (both ends of the onion can be colonized). Botrytis cinerea is much more visible, but regardless, these are two fungi that have expensive tastes.
In 2006, a fungus that made its first appearance in quite a while, was (Phoma terrestris/Pyrenochaeta terrestris) the cause of Pink root. In 2006 this disease was found in 3 fields and 7 different onion varieties. A disease that generally occurs at higher temperatures, (24-28°C is optimal, below 16°C little happens), this disease has been reported in southern states in the U.S. where it can be a severe problem. The damage to the roots can be quite extensive, and observations from the scouted fields this season, suggested that the disease severity increased (based on the number of roots infected per bulb) throughout the season. Symptoms above the ground appear very much like the plants are being impacted by drought, with leaf tip dieback, and a reduction in leaf number.
It is unclear what effect this disease had on sizing, and overall yield in 2006 (undersizing is the chief impact). The chief concern is attributed to root damage and reduction in leaf number (which influences bulb sizes, as bulbs are the bases of the leaves) (See picture below of root symptoms).
In general, insects were of less concern in 2006 than they are on a usual basis. Onion maggot flies (Delia antiqua) were not actively trapped in 2006 though onion samples were checked on a regular basis throughout the year. For the most part little damage attributable to onion maggot flies was found. The year was suitable for their development, but historically we seldom exceed the economic threshold and this year seems to hold well with that trend based on visual observation.
Onion thrips (Thrips tabaci) were an issue in 2006. Thrips tend to do very well in hot, dry weather and that is a suitable description for the weather conditions for 2006 on the whole. Numbers tended to peak around late July and into early August. Damage was quite evident on many varieties of onions (i.e. not green onions) that the tops are not valued on. Control measures were instituted because the thrips damage, while not to the marketable portion of the crop, was significant enough to reduce yields.
Potatoes:
As a general observation, it appeared numbers of Colorado potato beetle (CPB) (Leptinotarsa decemlineata) were up this year. That’s probably a bit of an illusion that can be attributed to lower than usual numbers in the very wet years of 2004-2005. CPB does not do particularly well in cool, wet conditions but does thrive in the type of year we had in 2006. To a small degree, growers were caught off guard by both the numbers and the speed with which they progressed through the juvenile stages.
Potato flea beetles (Epitrix cucumeris) continue to be a concern, though numbers remain relatively low. As a defoliator they often act in concert with CPB to reduce yields, but effective control measures continue to keep them in check and they are of little concern to table stock.
Aphids were very abundant this year, in potatoes and all other crops as well. Generally not a concern in table potato production, numbers were high enough this year to cause aphid holes, areas were plants collapse to do high levels of feeding by aphids. Buckthorn aphids, (Aphis nasturtii) and potato aphids (Macrosiphum euphorbiae) were the most abundant specimens collected. Less of a concern in table production, aphids were abundant and causing grief in processing production for a considerable period of time this year.
There were several reports of potato leafhoppers (Empoasca fabae) in processing potato fields. This is a bit unusual in certain respects. Typically when potato leafhoppers migrate into Manitoba we have ample warning from first alfalfa fields and then bean fields that tend to be congregated first. This year we had no such early warning indicators and the reports of leafhoppers were quite widespread to the west of the Portage la Prairie area. Each year, even in what could be considered a non-outbreak year we have fields that are sprayed due to exceeding economic thresholds and it looks like this year it was just a bit more widespread than usual.
For information on late blight of potatoes (Phytopthora infestans) please go the the Manitoba Potato News
website. There were cases of late blight this year reported in the area around Carberry.
Cole crops
This year would certainly qualify as a good year for insects. Most prevalent in people’s minds would be the imported cabbageworm or cabbage white butterfly (Pieris rapae), owing to the very visible presence of the adults flying around during the daytime. It was also an exceptionally good year for diamondback moths (Plutella xylostella) in some areas of the province as well and these are much more cryptic than the imported cabbageworms so they probably don’t get due credit for causing damage this year. We were fortunate though in that the cabbage loopers (Trichoplusia ni) did not make an appearance this year. Cabbage loopers are the most damaging of the three caterpillar pests and like the diamondback moth, are migrants from the US not overwintering here in Manitoba. 
The imported cabbageworm does overwinter here and if you recall the vegetable reports from late 2005, populations were high and significant damage was occurring in September.
Diamondback moths were present and causing feeding damage first this year. Early indications from the canola monitoring program were that these wouldn’t be a significant issue this year. However field scouting revealed that the numbers were actually quite considerable with upwards of ten caterpillars per plant. Several generations were experienced this year and moderate damage could be attributed to diamondback moths.
More significant than the diamondback moths were the imported cabbageworms (ICW). As mentioned previously, they overwinter in Manitoba and the numbers were high going into the fall of 2005. This translated into the high populations we experienced in 2006. At least three full generations were completed and considerable damage could be attributed to ICW caterpillars in late summer, including at least one commercial field being written off due to high levels of damage. We do have the expectation that natural enemies (predators, parasites, diseases) of this caterpillar will increase and the outbreak will end at some point in the not too distant future. As this pest overwinters here in Manitoba though, that may not occur soon enough and we may again experience problems with the pest in the next growing season.
Flea beetle (crucifer - Phyllotreta cruciferae and striped Phyllotreta striolata) populations have been quite low for the past couple of seasons owing largely to the very wet weather of 2004-05. Towards the end of this year we did note the presence of low numbers of flea beetles along field margins indicating populations are again building up. The last time that flea beetle populations declined significantly was around 1992 and it took several years for the population to return to high levels. As such, we are cautiously optimistic about flea beetle numbers for 2007 but will keep a close eye should they rebound quicker than expected.
Corn:
In 2006, sweet corn diseases did not consist of anything unusual by Manitoba standards, what disease was observed was of little significance. It was a good year crop wise with no major outbreaks of any disease reported. One oddity, would be the genetic stripe symptom observed early on in the season, which really makes for an interesting picture, but is of little concern (interestingly we had genetic stripe aka. “chimera” on onions (see July 14th, 2006 report, and on carrots (see July 30th, 2006 report) Common smut (Ustilago maydis), and corn rust (Puccinia sorghi).
were also observed this summer, but again levels were not of great concern. Late in the season, infection of plant tissues by species of Fusarium, were observed. At high levels this could be of concern if ears suffered high levels of damage. In 2006 Fusarium levels were very low, and of no concern.
From an insect standpoint we would consider 2006 to be a good year. European corn borer (ECB) (Ostrinia nubilalis) was present, as always, but numbers were generally low in the areas we scouted. This is not to suggest the pest wasn’t present in higher numbers in some areas not scouted; the pest is notoriously patchy in Manitoba, high in some areas, low in others. The greatest problem with ECB is a low consumer tolerance for the presence of worms in corn. Consumers have a zero tolerance policy for both worms and the use of pesticides it seems, but reality continues to rear its head at the most inopportune times.
Corn leaf aphid (Rhopalosiphum maidis) numbers were also down, contrary to what happened with almost all the other aphids. They were present and conditions were ideal for substantial population increases but they simply didn’t happen. We’ll attribute the lack of problem to the presence of suitably high numbers of biological control agents present (e.g. ladybird beetles/ladybugs) and control measures for ECB.
Cucurbits:
While powdery mildew (Erysiphe cichoracearum or Sphaerotheca fuliginea).was the most obvious disease, primarily on pumpkins, the damage was relatively minor. The damage to vine growth appeared minimal, and yield reduction did not appear to be of concern in our scouted fields. On pumpkins the chief damage is generally on the handles (the remains of the vine) and somewhat discoloured rinds.
Conditions for powdery mildew development on pumpkins generally occur late in the season, in Manitoba when the leaf canopy is well developed, which promotes humid conditions. In 2006, there were days in which there were significant changes in temperature from day to night, which were suitable for disease. The best way to manage this disease is use resistant cultivars, fungicides are available, but once an outbreak begins, control is difficult to impossible. High levels in 2006, do not imply high levels of powdery mildew in 2007.
Angular leaf spot (Pseudomonas syringae pvr. lachrymans ) developed quite rapidly on cucumbers, almost going unnoticed by our scouting program. This was probably the most damaging disease on cucumbers this year, and likely impacted yield, through development of secondary rots, and sunscald due to leaf loss, in addition to any direct effects of the bacterium. The disease was also observed on pumpkins in late July, but not further damage appeared to result in the scouted field.
We think it’s fair to consider 2006 as the year of the (striped) cucumber beetle (Acalymma vittatum). Typically in Manitoba we have some cucumber beetle present somewhere in the province but the distribution is almost always patchy. This year the beetle was present in almost every part of the province and in numbers higher than we’ve seen in at least 10 years. Feeding damage was fairly minimal, though obvious in some places. The larger concern for cucumber beetles is their ability to vector bacterial wilt. Fortunately there seemed to be very little disease present in the beetles this year and there were only a few cases of bacterial wilt being present in fields and even that was minimal and of little economic concern overall.
We do need to remember 2006 when spring of 2007 rolls around though as the beetles overwinter here in Manitoba and not far from the fields they were present in during 2007. Though disease levels were low this year, they may increase in the following year, so early and frequent scouting of fields prior to and during flowering is strongly recommended.
Carrots:
What a difference a year makes! In 2005, the leaf blight diseases ran rampant across carrot fields in August. In 2006, they were not even on the radar, and were not observed once, we can thank the dry, hot weather for that.
Forked carrots/tap roots were found as always, and it is still unclear what the cause is assuming there is only one cause in a field at any one time. Anything that damages the growing root tip can cause forking, Pythium have been implicated in other areas, along with soil compaction. In the majority of carrots collected this year, the forking began near the crown (top) of the tap root, suggesting the damage occurred early in the season. In the fields scouted this level held constant at about 12- 16% (3-4 carrots out of 25 collected each week)
Cracked taproots, were also collected in 2006. With the high temperatures throughout much of the summer, and the ongoing need for irrigation, this wasn’t a surprise. Cracked taproots can occur when periods of slow growth are followed by rapid expansion. This generally occurs when soils gets dry, followed by an influx of water. The cracks were by no means widespread in any of the scouted fields, which were irrigated as necessary to meet water demands. The rapid growth in the carrots this year both in terms of taproot elongation and leaf development (see graphs below) would put some stress on any carrots, particularly those which may have had some predisposing injury i.e. any sort of mechanical damage present.
Aster Yellow symptoms were observed in 2006 on carrots, parnips, and celery plants in Manitoba. Apart from the changes in foliage colour and the hairy root symptom, the carrots become quite bitter, and unmarketable. Once the phytoplasma has gained entry, there is nothing that can be done. Management of the vector, the aster leafhopper is imperative, whether the crop is carrots, celery, parsnips or a number of other crops.
Aster leafhoppers (Macrosteles quadrilineatus) remain a continual problem for carrot growers in Manitoba.
In retrospect, the population pattern wasn’t particularly unusual. Normally populations peak in August and September before naturally falling off. The population peaks observed a fairly normal pattern in 2006, though were a little earlier than usual probably owing to the much greater heat accumulation earlier in the season. Perhaps the most interesting aspect is that the peak Aster Yellows Index (AYI) values didn’t go as high as we normally experience. Manitoba often has AYI values peaking in values that reach into the thousands; the highest I can recall is a shade over 5000. Compared to that, an AYI of 500 doesn’t seem quite so bad. Regardless those values are both well above the economic threshold for resistant carrot varieties and can cause considerable economic losses if left unchecked.
During the year we had mentioned some potential damage to carrots that may have been caused by Lygus bugs (Lygus lineolaris and Lygus borealis). Following harvest there wasn’t any comment on difficulty in harvesting the carrots (damage present may have resulted in tops pulling away leaving roots in ground) so for now we’ll just adopt a wait and see attitude with respect to Lygus in carrots. They are known to be pests of carrot seed production but aren’t typically associated with conventional production. If indeed it was Lygus damage it’s probably not something we should worry about in years other than those where the canola harvest is exceptionally early as it was in 2006. The harvest/swathing of canola results in the Lygus bugs migrating out of the canola into other, still green crops and isn’t exclusive to carrots. Far from it the migration is well understood in a variety of crops including buckwheat and alfalfa, both preferred by Lygus over carrots.
|
|
|
As is the tradition, we explain how to calculate the Aster Yellows Index (AYI):
AYI is calculated by taking the number of leafhoppers and multiplying this by the infectivity level below. The migrating aster leafhopper population infectivity in Wisconsin is 3.5%. This is slightly higher than the normal 0-2%.
|
|
Crop | AYI |
| Carrots | |
| Resistant (eg. Six Pak II) | 100 |
| Intermediate | 70-75 |
| Susceptible (eg. Spartan Bonus) | 40-45 |
| Celery, Romaine | 30-35 |
| Head Lettuce | 20-25 |
| Prepared by: |
Brent Elliott Entomologist Ph. 204-745-5669
Philip Northover Pathologist Ph. 204-745-5694
Manitoba Agriculture, Food and Rural Initiatives
Soils and Crops Branch, Carman |
|
HomeContact
UsAbout
ManitobaDepartmentsLinksPrivacy