Aug 12, 2022 - MB Potato Report #13

Dr. Vikram Bisht
Potato & Horticultural Crops, Manitoba Agriculture

Rainfall was very sparse for the week (Aug 1-7) in Manitoba, and ranged from 1 mm to 11.5 mm in various weather station sites.

So far, the cumulative rainfall at many sites is now closer to 110% (Altona, Carman and St. Claude), while Austin, Bagot and Rivers are still around 170% of the 30-year normal (Table 1).

The crop water demand (CWD) for the week was similar to last week’s. The week’s CWD (slightly over 25mm or 1”) was much higher than the rainfall at all the sites in Manitoba (Table 1).  

Based on water holding capacity, the soil moisture in the top 30cm soil zone is optimal to dry in many potato grower areas (Fig 1). Irrigation was necessary to maintain 65-70% moisture (WHC) in most potato areas. At this stage of crop growth irrigation is important for bulking.

Temperatures for the week (Aug 1-7) with daytime temperatures in low 30sOC, and nighttime <10OC, were slightly cooler days and cooler nights as compared to last week (Table 1), which was good for tuberization and bulking.  

Accumulated Potato heat units (useful heat 7OC to 30OC) from June 6 –Aug 7 across the potato growing areas of Manitoba range from 510 to 532 (Fig 2). This P-Days value indicates that Manitoba potatoes are at peak bulking stage.  This is close to or just above normal so far, 105 to 115% of normal.

Tuber set in many fields is good and sizing is progressing satisfactorily (Fig 2 a,b), but in some fields impact of heat stress can be noticed in RB and some other varieties. There have been a few >30C days in previous weeks. 

Early fresh/table potatoes are being top-killed. Most fields with Group 4 herbicide injury symptoms seem to have overgrown the malformation.

Late Blight:
The 7-day cumulative DSVs (Fig 3) suggest low to moderate risk conditions for late blight disease throughout the province, if late blight inoculum was present.

Sporonado passive spore traps for late blight spores have been set up at 18 sites across MB to provide early warning of possible late blight risk.  

Seventh week’s (Aug 2-8) PCR test results for presence of Phytophthora infestans (Pi) spores are negative at all 18 of 18 site samples submitted (Table 2).

it is important to scout for late blight even in the absence of spore detections, especially in wind-protected areas of fields – near treeline, low lying areas or areas not covered by fungicide.  In Fraser Valley Sumas region Sporonado spore traps did not detect Phytophthora spores even though the late blight disease has been reported in the region for 3 weeks.

Late blight on tomato has been reported in eastern Ontario (Ottawa County):  July 26 report.

Blackleg disease continues to take down a few plants in various fields. Injury points on stems (ECB bore holes and hail damaged stems) are common entry points for the bacteria leading to aerial stem rot infections

Under the warm and humid conditions early blight disease is showing in more fields.  However, most fields of Russet Burbank have very minor incidence, as compared to varieties like Rangers and other early maturing varieties (Fig 4).  Lower canopy should be scouted for early blight.

Aphid & ECB monitoring:

Aphid counts have suddenly jumped up compared to last week. These numbers are higher than last year around this time.  Green peach aphid (GPA) was found in one field in a pan trap. Potato aphids (PA) continue to be trapped in all seed potato producing areas. The total “other” aphids are comparable to last year at this time (around Aug 8). Soybean aphids are also being trapped and are grouped with “others”.

GPA and PA are efficient vectors of PVY viruses and increasing trend in their numbers points to immediate control measures should be undertaken.  Along with regular “mineral aphid oil” sprays, insecticide targeting aphids should be used as a PVY management strategy. 

European corn borer ECB trapping has started to go down (Table 4), but there are reports of ECB stem boring from more fields, but no egg masses were seen. This year insecticide application was not needed in most locations. The ECB monitoring will end next week.


Colorado potato beetle young larval instars at different stages can be seen in some fields even after foliar insecticide application (Fig 5).

What is your DIAGNOSIS of the problem below?

After a few hard rains, this field was wet for a few days and the top foliage showed extensive yellowing with little if any veinal greenness (Fig 6 a, b).  One variety showed these symptoms, while another variety beside did not show yellowing.

The soil pH is 8.0+0.1, EC high (0-6” = 2.5 mmhos/cm, 6-24” = 3.67 mmhos/cm.

Soil test in Early August: All major elements were sufficient. S and B were sufficient. Mg, Zn, Fe, Mn and Cu were high.

Petiole test in late July (at the time of these symptoms) showed that S, K, B, Zn and Cu were deficient, while P and Mn were low. Ca, Na and Fe were high (Fig 7)

In 7-10 days, in the absence of any significant rainfall in the field, the new growth appears to be regaining green colour (Fig 7).

So what could have caused this severe chlorosis?

I will share the opinions and reasoning for the diagnosis sent by the audience.


This report is compiled using information from potato growers and agronomists. Please contribute if you have some information or photos to share.