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Bolting Resistance
Description
Sugarbeets are biennial and to flower must be photo-thermally induced. They
first require exposure to temperatures around 40° to 42° F, followed
by exposure to increasing day length (12 hours or more). Varieties vary in
their sensitivity to bolting so there is no fixed amount of exposure required
by all varieties. Easy bolting lines require only a few to 1000 hours of exposure
to low temperatures, while bolting "resistant" lines may require
2000 hours or more. Beets can de-vernalize when exposed to high temperatures.
In a normal climate year, spring temperatures are warmer and some inhibition
or reversal of bolting occurs as warm temperatures occur in May and June.
Development
During mild years, there can be less de-vernalization weather than normal, and a larger number of beets may bolt. Sugar losses are not large if beets can be harvested before seed stalks mature because much of the sugar content of the roots has been accumulated by the time bolting begins in late spring and beets have a large photosynthetic capacity.
Where there is a pressing need to establish Rhizomania resistance, some bolting resistance may be lost in the breeding process of new Rhizomania resistant varieties. It is difficult to advance a large number of resistance traits simultaneously in a sugarbeet breeding program. Breeders try to find the right equilibrium between bolting resistance and sugar yield per acre.
Managment
If bolted sugarbeets are allowed to mature, the sugar content of roots falls off, and viable sugarbeet seed is left in the field and field edges. Some of this seed can persist for years and will germinate as a weed in other crops or along field edges. Sugarbeets are not good competitors with other crops, but any that survive and become established may act as a reservoir for the yellowing viruses (Beet Yellows and Beet Western Yellows) and for Beet Mosaic Virus. The most serious disease is Beet Yellows Virus which can only infect seedling beets by being transferred by green peach and black bean aphids from nearby old, infected beets. For this reason, seedling beets growing in other crops or along field edges are a danger to the entire industry. Fields left to set hard seed can harm an entire region’s beet crop.
Aphids have an amazing ability to find host plants, even if they are small, or hidden in a crop like wheat or a hedgerow. It is important that bolted sugarbeets be prevented from forming mature seed. A good sugarbeet seed crop will yield about 4,000 pounds of seed per acre. There are about 40,000 seeds per pound so an unchopped field may produce 160-200,000,000 seeds.
The risk of mature (hard) seed formation and woodiness in beets becomes greater the longer the harvest season progresses. As beet stalks become woody, topping canopies prior to harvest results in some of the woody beets being pulled up and dislodged or cracked and broken with large pieces of root left in the ground unharvested. These losses can affect yield, perhaps significantly. So topping should be done when seed stalks have formed, but before they become woody.
References:
Sugarbeet Notes
By Stephen Kaffka, Department of Agronomy and Range Science
University of California, Davis
