Black Point - Screening eases the black point headacheOn-Farm Risk Assessment - Keep an eye on the triggers
GroundCover™ Issue: 53
Black point is not a disease. It is a degradation of the seed coat caused by enzymes and other chemicals in the seed coat that are triggered by humidity during the later stages of grain filling.
In Queensland, northern NSW and coastal portions of the northern WA wheatbelt, susceptible varieties are affected every year.
In extreme seasons this grain defect has been estimated to cost the grains industry more than $50 million.
For the past 10 years Dr Peter Williamson, of the Queensland Department of Primary Industries and Fisheries, has been trying to unravel the cause and controls of black point. He has screened well over 1000 lines of bread and durum wheats to assess their susceptibility to black point.
Bread wheat containing more than five percent black point or durum wheat containing more than three percent will be downgraded at receival because it results in black specks in the flour and discolouration of the end product.
Dr Williamson"s screening program has found that few of the current varieties have sufficient tolerance to black point if trigger conditions occur.
"Developing this screening program has resulted in the active rather than opportunistic screening for black point," he says. "Prior to this lines may have been rejected for black point but it was more by luck than judgment; our nurseries for this screening program are located in areas where susceptible lines are virtually guaranteed to develop black point."
As part of the screening process Dr Williamson has established three screening sites - at Bundaberg and Toowoomba in Queensland and Millicent in south-east SA. The geographical spread of these sites not only offers reliability of conditions that appear to be conducive to black point, it also offers protection against natural environmental variations, such as drought.
The benefit of having three geographically dispersed sites helps ensure the researchers are identifying a line"s genetic predisposition to black point, which could be masked by regional conditions in a specific testing year.
Dr Williamson"s screening program can be accessed by all breeders and the results for commercial varieties are incorporated into some state departments" variety sowing guides.
Much to Dr Williamson"s disappointment, black point continues to be incorrectly labelled as black tip in some of these lists. Other terms such as black ends, kernel smudge or kernel blight are also used to describe black point, but these only cause confusion, as they describe grain discolouration from several other causes such as fungal stains and bacterial and fungal diseases.
However, in Australia, these problems are rarely encountered and "black point" should be used to describe only those grains that become discoloured at the germ end and are otherwise healthy.
All plants in the screening nurseries are grown to maturity and each line or variety is harvested separately and the seeds checked for signs of black point.
Scoring points: Miriam Michalowitz is working with Weiss Enterprises to fine-tune a scanner that they have developed to count the level of black point in a sample. Photo: Peter Williamson
This is a labour-intensive task carried out by Miriam Michalowitz. Miriam is working with Weiss Enterprises to fine-tune a scanner that they have developed to count the level of black point in a sample. She hopes that the accuracy and consistency of black point rating will be greatly improved in the near future.
"We had hoped to develop a laboratory screening test based on the biochemical pathways involved in the development of black point. So far we have not been successful and instead are adopting the mechanical scanning system."
The specific environmental conditions which trigger black point have not been established but it is known to be associated with high humidity around grain maturity.
However, the genes which control black point have been located and markers for these genes have been developed by colleagues at the University of Southern Queensland.
It is thought that the genes which offer resistance are not dominant and there are several genes involved in the expression of resistance.
Using a set of full diallel crosses, Dr Williamson and colleagues are determining whether the presence of more than one of these genes increases the level of resistance. They are now experimenting with "pyramiding" resistance genes to try to strengthen the level of resistance.
"In this situation markers help us identify the gene but they cannot give us any indication of the level of resistance inferred by that gene. This can only be tested by developing crosses and testing them in the screening nurseries."
Black point is a cosmetic rather than a chemical defect which has no impact on yield or intrinsic quality, but it still causes a severe headache to grain marketers. With careful milling the seed coat can be removed to prevent the black specking of the flour, but that increases the end users" cost.
It is Dr Williamson"s aspiration that no highly susceptible bread wheats will be released in the future - and his research is helping to achieve that aim.
For more information:
Dr Peter Williamson, 07 4639 8888, Peter.Williamson@dpi.qld.gov.au
GRDC Research Code: DAQ00045, program 1
Listen also: Harvest Radio - Weather Damage
The appearance of sprouting-tolerant wheat cultivars will depend on wheat-breeding companies recombining dormancy with the large number of agronomic, disease resistance, adaptation and quality traits required in any successful commercial cultivar. In the meantime grain growers can use the improved understanding of the triggers for sprouting, together with local weather forecasts, to assess sprouting risk and potential for downgrading.
Where the total rainfall in the 10 to 15 days prior to crop maturity exceeds 10 millimetres the grain will be more susceptible to any further rainfall after crop maturity. Tolerance to sprouting declines more rapidly in high temperatures and is reduced if symptoms of black point are present.
"If, based on these criteria, the crop is considered to be at high risk, then growers should aim to optimise harvest and, if possible, consider options such as early harvest followed by aeration or grain drying in order to minimise the chance of costly downgrading," says Dr Mares.
"Although there is no genetic link between black point and sprouting, black point symptoms are associated with an increased germination rate in harvest-ripe grain. Black point is a visible defect and it can help alert growers to the greater risk of sprouting."
As the crop passes harvest-ripeness, the potential for sprouting during any particular rain event increases steadily. For sprouting to occur, the grain needs to be wet (>30 to 35 percent moisture) for at least one to two days. Consequently, sprouting is favoured by rain combined with humid, cloudy and wind-free conditions or, of course, continuous or frequent showers.
For more information:
Associate Professor Daryl Mares, 08 8303 7480, email@example.com
GRDC Research Code: UA00039, UA00034, program 1