Jun 26, 2020 - MB Potato Report #6

Dr. Vikram Bisht

Potato & Horticultural Crops, Manitoba Agriculture

The cumulative heat has come closer to the normal in potato production areas, ranging from 90 to 100% of the normal GDD accumulation.  http://www.gov.mb.ca/agriculture/weather/pubs/percent-normal-gdd.pdf

The P-Days (potato heat units equivalent) is nearly 100% or just above the normal for this period of time (Fig 1).  https://www.gov.mb.ca/agriculture/weather/pubs/percent-normal-p-day.pdf

The low rainfall for the week was generally around 3 to 8 mm; and ranged from 1.4 mm in Carberry to 22.7 mm in Treherne. Accumulated precipitation has been below normal (ranging from 35 to 70% of the normal rainfalls) up until mid-June for the potato growing areas (Fig 2, Table 1). 


The crop is in various growth stages – from 24” height in late April planted fields, to just emerging plants in fields planted in early June (Fig 3a & 3b).

Most of the herbicide applications and hilling are finished. Winds have been rather strong lately (Fig 4a).  Such strong winds have caused abrasion injury of young sprouts (Fig 4b & c) in sandy fields and could make the plants prone to early infection with black dot or even blackleg (Fig 4d). 

In cool and wet soils of early planted fields, the emergence was slow (up to 5 weeks) compared to later planted fields which emerged quicker - ranging from 3 to 2+ weeks. Blackleg disease affected plants (Fig 5) are being noted in more fields, mostly those which were planted early in cool wet soils. The infected plants show up as wilting (5a, b), yellowing, very slow emerging plants (5c, d), sometimes individually or sometimes in a group (5e, f).

Late blight Forecasting:

DSVs:  Currently, the cumulative Late Blight Disease Risk Values (DSVs) are very low (Fig 6), and there was zero risk added in the last week, due to warm and rather low rains.

Late Blight Spore Trapping:  Passive spore traps (Sporonado) were set up last week in 6 sites (Shilo, Carberry, Melbourne, Treherne, Carman, Winkler) across MB, and the spore trapping cassettes were sent for testing for Phytophthora infestans and Alternaria solani.  All traps tested negative for both pathogens. 

To keep the risk low, it is important to dispose off the cull potatoes in an appropriate manner so volunteers on the cull piles do not become source of inoculum.

Aphid monitoring:

Virus infected plants can now be easily seen (Fig 7).  For seed growers, 6-8” size plants are best time / stage for rouging. Once the plants close-in within rows, some of the small or late emerging plants may stay hidden. Cloudy days are best to differentiate the slight variations in color or mosaic patterns on the foliage. Such infected plants can be critical source for further spread within a field.

Aphid suction traps were set up in 8 seed potato fields. Aphids were trapped in 2 of 8 locations and were not classified as significant vectors of potato mosaic viruses. In Carberry: 3 aphids and Winkler 2 aphids, were trapped. Green Peach Aphid and Potato Aphid were not trapped.

European Corn Borer monitoring:

ECB pheromore Delta-traps were set up in 10 fields across Manitoba. None of the traps had any adults trapped after 1-week.  The crops have been about a week behind last year and so it may partly be the reason for zero count. The monitoring will continue for at least 3 more weeks.

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