Jul 22, 2021 - MB Potato Report #11

Dr. Vikram Bisht
Potato & Horticultural Crops, Manitoba Agriculture


The week of July 12 -19 was again too warm for potatoes in most areas, reaching up to 30-34OC for 2 to 4 days in the week (Fig 1).

The cumulative Potato heat Units, P-Days (between 7C and 28C) are 350 – 376, which is about 100 to 110% of normal for this time of the season. https://www.gov.mb.ca/agriculture/weather/pubs/percent-normal-p-day.pdf.   

The soil moisture in top 30 cm zone has continued to lower further, and more area is 20-40% of saturation levels. The area under “dry” conditions has increased slightly over last week. https://www.gov.mb.ca/agriculture/weather/pubs/soil-moisture-30cm.pdf

Crop water demand for the week in selected sites has been >36 to 53 mm (Table 1) but the rainfall has been scattered and highly variable, ranging from 0 to 24.5 mm).  Without irrigation, 30cm soil moisture in Shilo, Treherne, Holland, Portage, Winkler, Morden are categorized as “dry”, and around 20-40% of the soil saturation levels.

There has been very little rain (0 – 24.5 mm) in the week, July 12-18 (Table 1), and in many potato areas show mostly 50-85% of the normal cumulative precipitation for the period.  Winkler had very high temperatures and <50% of normal rains.

The week, July 12-18 was about as warm as the previous week (Jul 5 – 11). The temperatures are forecast to be in mid-20s in coming few days. (Manitoba - Weather Conditions and Forecast by Locations - Environment Canada).

‘* Crop Water Demand: cwd (mbpotatoes.ca)


Late Blight:

The accumulated late blight disease risk (DSV) values at various sites across MB have increased, and range from 11 to 24.  The 7-day cumulative DSVs range from 5 to 11 (Fig 2) in potato growing areas – and suggest low to moderate risk for late blight if inoculum is present.  Late Blight (mbpotatoes.ca)

Late Blight Spore Trapping:  Passive spore traps set up at 15 sites (Shilo, Carberry, Glenboro, Melbourne, Treherne, Bagot, Portage, Carman, Winkler) across MB, and the spore trapping cartridges were sent for testing for Phytophthora infestans.  

Phytophthora spores were not detected in the spore traps at any site (Table 2).

Even though the macro-climate may not be very conducive to late blight disease or spread, it is possible to have micro-climate allowing disease development in well-irrigated fields, which are wind protected. It is extremely important to scout for spots in fields, which stay wet overnight after irrigation and are wind-protected.

Aphid monitoring:

Overall, in 8 seed fields the numbers increased slightly as compared to the previous week. In week 5, no Green peach aphids but a few Potato Aphids were trapped. The “other aphids” numbers have increased the southern region over the high numbers of last week. In the western seed area, the aphid counts remained similar to last week. Potato aphids increased at one site, but generally reduced at most sites.  At one site, apparently the suction trap may not be working, resulting in zero aphid trap (Table 3).

Hot and dry area of MB south seed area may be encouraging aphid multiplication. In central seed region, the sites show stable or increased numbers over previous week.

It may be worthwhile to continue with Aphid Crop Oil application to keep the virus transmission down, especially in fields close to high virus fields.

European corn borer:

ECB pheromone traps have been set up in 13 locations in areas where infestations may get high. This year, along with the normal Iowa strain (common in Manitoba), we are also testing NY strain pheromone in some fields, to see if we have ECB strain NY in MB.

The trap catches show a surge in numbers of ECB catches in the western parts of Manitoba, in traps with Iowa strain lures (Table 4). ECB adults are being trapped in the sticky traps with NY strain lure; these adults will be tested to confirm strain identification.  

Currently, the “Live traps” (Heliothis type mesh) are trapping fewer ECBs than the delta traps set up in the same fields in Carberry and Carman. Low numbers may be a reflection of low ECB numbers in the area. 

Agronomists are reporting eggs and young larvae (Fig 3 a, b) and stem borer injury (Fig 4 a, b) from many fields, mostly in western Manitoba area. If numbers are high then it may be a good time to spray when the larvae are young and more eggs have hatched. There is about a week’s lag to get to 50% larval numbers after first egg masses are seen. The possibility for higher incidence of ECB injury exists.

As per Ontario IPM guidelines – there is no threshold number given for potatoes. Research from PEI suggests 2 egg masses/ 10 plants as action threshold. Secondary bacterial stem-rot issues due to borer injury should be taken in to account. Research in US found minor direct economic loss due to ECB stem injury (Kennedy, 1983. Journal of Economic Entomology 76:316-322), but loss of yield and quality from bacterial soft rot could be important. Direct yield reduction due to ECB may happen if main stem get affected early in season, leading to vine death or stem rot.

Colorado potato beetle:

The expected annual surge in Colorado potato beetle numbers has not been reported so far! Is it the heat or the populations are suddenly sensitive to the usual insecticides.  It is still important to monitor areas in fields (especially edges) for CPB activity. 

Early Blight:

The cumulative P-Day units now >350, had crossed the 300 mark about a week and half ago, and now early blight is seen in a few more fields (Fig 5).  However, most fields are not showing much early blight even in the lower canopy, and protectant fungicide would be sufficient.

Ranger Russet, early maturing or susceptible varieties should be scouted. If the early blight spots are moving to the mid-plant area and the variety is susceptible, fungicide application may be considered. 

Due to generally dry conditions, there is limited risk of the disease taking off. Humid conditions in the last 2-3 days may favour disease progress in susceptible varieties.

All information and other reports will be available at http://www.mbpotatoes.ca/index.cfm

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