PestFax No.18 06/09/13 - Department of Agriculture and Food - wa ...

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Pseudo black chaff A grower at Esperance reported finding dark-brown to black discolouration of the upper stems below the heads on his advanced Mace wheat crop which is at the milky dough stage. A search of information lead the farmer to suspect “pseudo black chaff” as the reason for the discolouration, which was confirmed by further discussion with Plant Pathologist Ciara Beard. The crop received an application of fungicide 3 weeks ago reducing the risk of septoria glume blotch infection which can have similar looking symptoms. Ciara says that pseudo black chaff (PBC) is not a disease. It is also known as False Black Chaff or Melanism, and is a physiological condition that is associated with the presence of the stem rust resistance gene Sr2 which is used as one of the primary sources of stem rust resistance in a number of our current varieties. It is an over expression of melanoid pigments in the plant causing a dark-brown to black discolouration of heads and exposed stems below the head (see pictures below). It typically causes blackening of glumes and/or upper stems. There is some suggestion that it can compromise yield in affected varieties. Symptoms generally start to appear post flowering and become more obvious under warm humid conditions as the crop matures. It is believed that environmental conditions, such as prolonged wet weather, high humidity and high levels of UV radiation induce the symptoms which are sporadic from season to season. Importantly there are no known control options for PBC. Varieties commonly grown in WA that are known to carry Sr2 and show varying degrees of PBC are: Carnamah, Derrimut, Ellison, Eradu, Machete, and Sentinel. Occasional cases of PBC have been reported in WA Carnamah crops in previous seasons. It is important to distinguish PBC from other causes of upper stem /glume discolouration such as Stagonospora (Septoria) nodorum which may be managed with fungicide. Septorias typical dark brown 'long patches' with narrow yellow margins may be distinguishable with the naked eye, particularly if the disease is also present on the leaves of the crop. However if in doubt, send suspect samples to AGWEST Plant Labs for diagnosis. Native budworm and DBM Farmer Peter Cripps reports finding 9 budworm caterpillars per 10 sweeps in a lupin crop west of Binnu, another farmer north of Wubin has found 1 caterpillar (18mm long) in 5 lots of 10 sweeps in a chickpea crop. Ty Henning (Tek Ag) is also beginning to see native budworm caterpillars appearing in crops. This week he found 2 caterpillars per 10 sweeps in a canola crop near Wongan Hills, between 2 to 12 budworm caterpillars per 10 sweeps in a crop near Beacon and 1 to 2 budworm caterpillars in 10 sweeps in crops in the Badgingarra/ Dandaragan area. Ty also commented that currently the caterpillars are less than 10mm, but as their size increases over coming weeks they will increasingly cause more damage to crops. 4
Results from moth trapping of native budworm adults can be found in the document: Native Budworm Moth Trap Numbers 2013. Large numbers of moths flying into cropping areas does not always equate to large numbers of grubs. This is partially due to the effects of predators, parasites and weather. Eggs laid by moths will take at least two weeks before they grow to a size of approximately 5 mm and can be detected whilst sweep netting crops. It takes about 7 weeks from the egg stage to reach the fully-grown caterpillar stage of about 40 mm long, given favourable temperatures. The last two growths stages (5 th and 6 th instars, which are >25 mm in length) are responsible for eating over 90% of their total food consumption. The ideal time to monitor budworm grub numbers is when pulse crops such as field peas, chickpeas and faba beans first begin flowering and pod development. Canola and lupin crops differ from the pulse crops in that native budworm caterpillars will only cause damage to pods close to plant maturity when these crops are starting to dry off. For more information, see the Management of Native Budworm Farmnote. Diamondback moths While sweep-netting crops for native budworm caterpillars Ty also detected diamondback moth (DBM) caterpillars. He found 6 - 20 DBM caterpillars per 10 sweeps in canola at Wongan Hills and 3 - 50 DBM per 10 sweeps in a crop at Beacon. He suggested that the recent wet weather in the Badgingarra / Dandaragan area has helped to kept the DBM population in that area to relatively low levels. Flag smut Cara Allan (UWA) reports finding sporadic patches of flag smut in a wheat crop near Cunderdin. Flag smut is identified by long grey-black raised streaks on leaves, leaf sheaths and occasionally stems. When you rub your hand against them the powdery grey-black spores rub off easily. Leaves are often twisted and split lengthways, heads may not emerge or may produce poor grain. Plant Pathologist Ciara Beard says that spores of the flag smut fungus are carried on seed and in soil and can be wind spread, the spores can survive in soil up to seven years infecting successive wheat crops. Infected grain will carry the disease into the next crop so new seed should be obtained for sowing or infected seed should be treated with a registered fungicide seed dressing. Resistant varieties are available. This disease is well-managed by fungicide seed dressing and occurs only sporadically, usually following successive plantings of untreated susceptible varieties. In paddocks contaminated with flag smut, use clean fungicide-treated seed of resistant varieties to reduce disease risk. For further details see the Biology identification and management of cereal smut diseases. Blue oat mites David Pfeiffer (Synergy Consulting) reports that he was amazed to see very large numbers of blue oat mites on the leaf blades of a Mace wheat crop growing near Tammin. Hundreds of the mites were found feeding on some of the upper leaves creating yellowish patches and removal of the waxy cuticle by their sucking action. The paddock has not received an insecticide application this season and was sown to canola in 2012. Clover scorch Steve Poole (Landmark) reports that a few farmers in the Albany and south coastal agricultural areas have been finding damaging levels of clover scorch infections in some of their pasture paddocks. Clover scorch disease is caused by the fungus Kabatiella caulivora which affects several subterranean clover varieties in the high rainfall areas of WA. The fungus most commonly infects the area where the leaflets join the leaf stalk causing “turning” of the leaves. Brown ‘grooved’ lesions can appear on the stalks and stems cutting the moisture supply and ultimately leading to wilted ‘scorched’ leaves. Control Clover scorch can be managed by using resistant varieties and grazing throughout the year. Fungicides can be a viable short-term control measure in clover-dominant hay and seed crops of susceptible varieties where heavy grazing is not possible. For more information, see Farmnote ‘Clover scorch disease of pasture legumes and its control.” 5
Page 1 and 2: PestFax Issue Number: 18 6 Septembe
Page 3: Wheat diseases Mark Ladney (Ravensd

PestFax No.18 06/09/13 - Department of Agriculture and Food - wa ...

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