Canola Flowering and Fungicide Application Timing
A-1208, June 2001
Terry Gregoire, Area Extension Specialist/ Crop Production
Duane Berglund, Agronomist
Art Lamey, Plant Pathologist
No canola varieties are resistant to sclerotina stem rot (Figure 1). Fungicide application offers the only means available to reduce stem rot when the environment favors infection. North Dakota currently has two fungicides available for control of stem rot. Both require timely application to be effective. The Ronalin EG label recommends application at 20 percent to 50 percent flowering. The Quadris label recommends earlier application at 10 percent to 25 percent bloom. Studies in the early 1980s in Alberta, as well as in other parts of western Canada and North Dakota, suggest economic returns to fungicide application may occur when at least 20 percent of B. napus (Argentine) plants are infected. The use of fungicides not only increases yields but also reduces dockage due to sclerotial contamination of the seed and small shriveled seed.
Figure 1. Sclerotinia disease cycle. (9KB color diagram)
Days to flower after planting varies by season and variety or hybrid. Data from the North Dakota State University Langdon Research Extension Center shows flowering may begin as early as 38 days (1997) or as late as 58 days (1999) after planting. The average days to flowering at Langdon is 49 to 55 days after planting.
Flowering begins with the opening of the lowest bud on the main stem and continues upward with three to five or more flowers opening per day. Flowering at the base of the first secondary branch begins two to three days after the first flower opens on the main stem. Under reasonable growing conditions, flowering of the main stem will continue from 14 to 21 days for both species. Full plant height is reached by peak flowering.
Flowers begin opening early in the morning and, as the petals completely unfold, pollen is shed and dispersed by both wind and insects. Flowers remain receptive to pollen for up to three days after opening. If favorable warm, dry weather occurs, nearly all the pollen is shed the first day the flower opens. The flower partially closes in the evening and opens again the following morning. Fertilization occurs within 24 hours of pollination. After pollination and fertilization, the flower remains closed and the petals wilt and drop two to three days after the flower opened. The young pod becomes visible in the center of the flower a day after petals drop.
During flowering, the branches continue to grow longer as buds open into flowers and as flowers develop into pods. In this way the first buds to open become the pods lowest on the main stem or secondary branches. Above them are the open flowers, and above them the buds which are yet to open. All of the buds that will develop into open flowers on the main stem will likely be visible in B. napus within three days after the start of flowering, and within 10 days in B. rapa (Polish).
Bloom Stage Identification and Fungicide Application
If you decide to spray for stem rot control, you must determine the correct
bloom stage at which to spray (Table 1). Sample several plants over the field
and count the number of open flowers. One way to check for bloom stage is to
find the main stem, remove the secondary branches, and count only open flowers
on the main stem. Generally, it takes a crop two to four days, depending on
the weather, to move from first flower to 10 percent bloom, one to two days
from 10 to 20 percent bloom, and one to two days from 20 to 30 percent bloom.
The number of opened flowers on the main stem indicates the bloom stage.
Table 1. Identification of Flowering Stages of Canola ----------------------------------------- % Flowering Number of Open Flowers Stage on the Main Stem ----------------------------------------- B. napus B. rapa Argentine Polish 10 at least 10 6-7 20 14-16 10-12 30 at least 20 14-16 -----------------------------------------
At 30 percent bloom, a crop of B. rapa may appear to be in full flower due to many open flowers on the developing secondary branches. At this stage of flowering there will be little petal fall unless there were some very hot days after first flower.
A crop in the 30 percent bloom stage will have few pods set. With aerial application, the best control will be achieved by spraying before petals begin to drop and before pods set.
Fungicides provide protection to the plant for at least nine days when applied at the 20 to 30 percent bloom stage. This is the period when the most damaging infections are usually initiated and when the crop canopy still allows fungicide to penetrate. Ground application of fungicides will achieve better penetration of the crop canopy and make later spraying of the crop feasible.
At 50 percent bloom, a crop of B. napus will be at maximum color development. It will still have a few unopened buds at the top of the main stem and the lower pods will be elongating. However, at 60 percent bloom, all of the top buds will have opened and it's too late for fungicide application.
Canadian data shows that waiting until full bloom reduces fungicide effectiveness by ground or aerial application. If there are more than four pods set on the main stem of the majority of plants, fungicide application may not be economical. Because of the very short time between first flower and 30 percent bloom, you should plan application strategy as early as possible. Flowering can progress rapidly and application ideally should be made before the first petals fall. Early scouting and proper flowering stage determination will help ensure a proper application decision.
Bloom Stage Identification
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 10% Bloom (16KB color jpg) |
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 20% Bloom (35KB color jpg) |
 20% Bloom (12KB color jpg) |
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 30% Bloom (41KB color jpg) |
 30% Bloom (16KB color jpg) |
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 40% Bloom (45KB color jpg) |
 40% Bloom (17KB color jpg) |
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 50% Bloom (30KB color jpg) |
 50% Bloom (22KB color jpg) |
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 60% Bloom (34KB color jpg) |
 60% Bloom (21KB color jpg) |
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Photos courtesy of BASF
A-1208, June 2001
