Cereal Leaf Disease Update SA 2009 -

| Date: 03 Mar 2010

Cereal Leaf Disease Update SA 2009
 
Hugh Wallwork, SARDI
 
 
Variation in net form net blotch (NFNB)
Variation in NFNB is on the increase and making resistance ratings for varieties more complicated. After the demise of Skiff in 1999 only low levels of the disease were observed in barley crops, mainly on Barque on the lower Yorke Peninsula, and apparently only one pathotype was present. At the end of 2009 we have identified at least five strains with quite different patterns of virulence based on adult plant testing although two of these were most probably quite localised. The principal changes in virulence observed are:
·          Increased virulence on KeelA, which first appeared in 2007 and is now common throughout the Mid and Lower North
·          A Maritime virulent strain that swept down the West Coast in 2009 and caused significant damage to MaritimeA crops across the Eyre Peninsula and lower Yorke Peninsula
·          A new pathotype collected from a hotspot in a FleetA crop at Ungarra has shown increased virulence on Fleet, KeelA and CommanderA but is not virulent on Maritime or BarqueA.
·          A further pathotype, picked up in a barley trial at Bordertown has shown increased virulence on Keel and SchoonerA.
 
Table 1. Results of virulence testing of selected isolates on adult plants grown in controlled environment growth rooms
                                                                  Isolates
 
Mallala
Crystal Brook
Urania
Ungarra
Bordertown
 
Skiff
Keel
Maritime
Fleet
Fairview
 
1998
2009
2009
2009
2009
Barque
S
S
MR
MR
S
Keel
MS
S
MS
S
S
Maritime
R
R
S
R
R
Fleet
R
R
R
MS-S
MR
Schooner
MS
MR
MR
MR
S
 
Additional minor differences have been observed between a number of isolates that are virulent on Maritime and Keel but this may be an artefact of adult plant tests run in growth rooms which can show some differences not apparent in the field. Hindmarsh for example seems to perform worse in growth room tests than it does in the field. A large number of isolates were collected in 2009 and a comparison of these on the principal varieties grown in SA is ongoing in SARDI.
 
Given that NFNB goes through a sexual stage, there is every probability that we will continue to see new virulence combinations in the future in regions where these pathotypes overlap. Fleet or Schooner crops are not expected to show much susceptibility in 2010 expect in limited areas where virulence was observed in 2009 but growers will need to be aware that this situation could change quite rapidly because of the potential for the pathogen to change.
 
Given the amount of NFNB in crops in 2009 there will be high levels of inoculum in barley that are virulent on Maritime and Keel for the start of the 2010 season. This will lead to earlier and high levels of initial infection of crops and, depending on seasonal conditions, may require much earlier crop sprays than in 2009.
 
Seed tests on infected samples from commercial Maritime and Keel crops with NFNB infection have shown that few of them are likely to lead to seed borne transmission of the disease to young crops. Considering the high levels of stubble borne inoculum that will be present in SA in 2010, seed treatment for NFNB may provide little benefit this season. Seed transmission requires infection of the embryo and not just the husk but the reasons for the low level of seed transmission in 2009 samples is not yet apparent.
 
Variation in barley leaf rust
Barley leaf rust was widespread in SA in 2009 with perhaps two or more different sources of initial inoculum. A principal early source was rust that developed on the alternate host Star of Bethlehem on the Lower Yorke Peninsula. Rust from this source rapidly spread east and northwards and resulted in widespread use of foliar fungicides on the Yorke Peninsula.
 
At least two different populations of leaf rust with contrasting patterns of virulence were observed in NVT trials on the Yorke and Lower Eyre Peninsulas. Across the Yorke Peninsula the varieties Buloke, Commander, Flagship, Fleet, Keel, Schooner and SloopSA all showed susceptible to very susceptible reactions in trials at Arthurton, Brentwood and Warooka. However at Cummins a different rust population was observed so that Buloke, Commander, Flagship and Fleet showed useful partial resistance. These resistances are provided by minor adult plant resistance genes which are not detectable in seedling tests; hence they are not used in pathotyping of strains. These differences within the rust population therefore go unnoticed in annual surveys conducted by the University of Sydney but can lead to very significant differences in crop damage.
 
The occurrence of the Star of Bethlehem in paddocks on the Lower Yorke Peninsula means that barley leaf rust goes through sexual recombination each year leading to a diversity in rust strains that is not observed to the same extent in the wheat rusts.
 
Table 2. Leaf rust disease ratings from NVT trials on the Yorke and Eyre Peninsulas in 2009
 
                                                                                NVT trial sites
 
Arthurton
Brentwood
Warooka
Cummins
Barque
MS-S
MS
S
MS
Keel
VS
VS
VS
VS
Schooner
S-VS
S-VS
VS
S
Sloop SA
S-VS
S
VS
S-VS
Buloke
S-VS
S-VS
S-VS
MR-MS
Commander
MS-S
S-VS
S-VS
MS
Flagship
S-VS
MS-S
S-VS
MR-MS
Fleet
S
S-VS
S
MR
Hindmarsh
MS-S
MS
MS-S
MR-MS
 
 
Notes on other foliar diseases
Frequent rains in spring meant that conditions were very favourable for the development of scald in barley as well as yellow leaf spot and septoria tritici blotch in wheat. Inoculum of each of these pathogens will be present at higher levels in stubbles for 2010. However as there were no confirmed sightings of septoria in 2009, this pathogen is unlikely to be a problem this coming year.