Cereal disease update SA 2011

Cereal disease update SA 2011

Hugh Wallwork, SARDI

White grain
At harvest in 2010 a number of growers in the north eastern Eyre Peninsula and Far North regions had deliveries of wheat rejected due to white grain. Two samples of rejected grain were tested and a fungus very similar to Botryosphaeria zeae was identified. No Fusarium was found to be involved although testing of a wider range of samples will be undertaken when they are received. Botryosphaeria was first identified in wheat and barley heads in Queensland in 1999 when up to 30,000 tonnes of grain were found to be contaminated on delivery. The principal cause for concern of this disease is that it could be confused with head scab caused by Fusarium which produces toxins dangerous for human and animal feed. A study on weaner pigs fed with white grain infected with Botryosphaeria indicates that there are no toxin issues associated with this fungus (Kopinky and Blaney, 2010).

The wet conditions in 2010 almost certainly promoted the high incidence of the fungus. It is assumed to have arrived from the Northern Region by wind blown spores either in 2010 or perhaps earlier but lay undetected until weather conditions were favourable for severe infection.

The white grain fungus survives in cereal stubble and can produce both sexual and asexual spores which may disperse over large distances. The fungus is therefore likely to remain a concern for the future although severe problems are only likely to occur where wet springs promote infection late in the season. Tests are underway in SARDI to see whether there are any differences between wheat and barley varieties in the degree of susceptibility.

Powdery mildew in wheat and barley
Powdery mildew developed more severely on Wyalkatchem crops on the Eyre Peninsula than previously observed. Some Gladius crops were also affected but not as severely. The late damp conditions caused severe head infection and led to widespread use of fungicides particularly on the Lower Eyre Peninsula. Given the ideal conditions for powdery mildew and the large areas sown to susceptible varieties, the fact that barley mostly escaped infection demonstrates the effectiveness of seed treatments when practically all crops are treated. Where a few crops are not treated these provide a breeding ground for the fungus and can lead to severe epidemics once the effects of seed treatments have worn off. Continuing widespread use of seed treatments will also greatly reduce the risk of fungicide resistance emerging, an increasing problem in Europe and Western Australia.

Stem rust
Stem rust was observed on a very early sown crop of Kite wheat at Baroota in the Mid North. Kite was released with the stem rust resistance gene Sr26 which is effective against all strains of stem rust. After 44 years of growing Kite at Baroota it appears that the Sr26 gene, which has a yield reducing effect, has been selected out of the seed rendering these particular crops of “Kite” susceptible to stem rust. Similar selection along with seed mixing has led to nearby crops of Blade, also released with Sr26, also becoming susceptible to stem rust. We think that only a few of these crops were responsible for the spread of stem rust through the Mid North and Mallee and also to the eastern part of the Eyre Peninsula during late September and October. This should be a warning against the use of susceptible cultivars by even a minority of growers although in this instance the growers justifiably thought they were growing resistant varieties.

Leaf rust in wheat
Leaf rust was observed on the Eyre and Yorke Peninsulas from August onwards. Although somewhat cool, the conditions were more favourable for leaf rust than stem rust and had susceptible varieties been grown then a significant epidemic is likely to have occurred. Because almost all crops grown in SA are now at least only moderately susceptible (MS) the development of leaf rust was effectively suppressed. Growers are therefore encouraged to maintain this level of resistance and avoid adopting varieties more susceptible than GladiusA and YitpiA.

Yellow leaf spot
The wet spring of 2010 will have increased the levels of the yellow leaf spot fungus (Pyrenophora tritici-repentis) in cereal stubbles. It is to be expected therefore, that with this increased infection and higher quantities of stubble, yellow leaf spot could be particularly damaging in 2011. The actual level of infection will however also depend on humidity levels in seedling crops and on the variety sown. Growers should take particular care to select more resistant varieties if they plan to sow wheat into wheat stubbles in 2011.

Net form net blotch (NFNB)
A testing service for seed borne transmission of net form net blotch (NFNB) is available for barley seed from around South Australia. This is a free service provided to help identify the risks of early infection with NFNB in crops in 2011. About 100 g of seed is required as any samples with high levels of seed borne transmission may also be used for seed treatment efficacy testing.

All samples tested in 2010 proved negative for seed borne transmission probably because of dry conditions during flowering in 2009. With moist conditions during flowering in 2010 it is possible that infection of barley embryos occurred and this could lead to early seedling infection in 2011.


Leaf yellowing and death in wheat
“Frame yellows” which is not a disease but is often confused for one was particularly noticeable in many crops of Yitpi, CorrellA and AxeA during August. The yellows symptoms are more prevalent in wet winters and the cause remains elusive. In October a similar but different yellows condition became apparent in Gladius crops. Again the cause is unknown.
Another different but widespread yellowing and death of wheat leaves occurred in the Keith-Bordertown region and into Victoria during October. The symptoms were observed in all wheat varieties and were similar to barley yellow dwarf virus (BYDV) but this is not thought to be the cause owing to a lack of aphids and negative antibody tests

References
Kopinky JS and Blaney BJ (2010) Nutritive value and non-toxicity of Botryosphaeria zeae-infected wheat for weaner pigs. Journal of Animal Physiology and Animal Nutrition 94: 44-54.

Hugh Wallwork
SARDI
Ph: 08 8303 9382
Email: hugh.wallwork@sa.gov.au