Importance and control of spot form of net blotch (SFNB) in Victoria

| Date: 20 Jun 2008

Mark McLean and Grant Hollaway
Department of Primary Industries, Horsham, Victoria

Supervisor: Professor Barbara Howlett
University of Melbourne


Take Home Messages
• Spot form of net blotch (SFNB) has become the most prevalent foliar disease of barley crops in Victoria. Surveys have shown SFNB to be present in approximately 95% of barley crops.
• Prevalence of SFNB is favoured by the widespread cultivation of susceptible varieties (eg. Gairdner) and stubble retention practices.
• Yield loss from SFNB infection may only occur in a low proportion of infected paddocks where conditions are favourable for disease development.
• Growing varieties with resistance is the most effective method of control for SFNB
• In the absence of resistance, a single application of foliar fungicides between growth stage Z31 and Z39 can also provide effective control of SFNB.

Background

Spot form of net blotch (SFNB) is caused by the fungus Pyrenophora teres f. maculata and is a major foliar disease of barley worldwide. Variable effects on barley grain production have been reported, with both significant and negligible grain yield loss recorded in its presence. Implications of SFNB infection on barley production in Victoria is not understood as no studies into the epidemiology or control of the disease have been documented since its first detection in the early 1990s.

The aim of this study was to establish the importance of SFNB to Victorian barley production and identify control strategies for its management. In order to do this, a survey of barley crops in Victoria and field trials at two sites were carried out in 2007. Results were combined with previous studies conducted in 2003, 2004 and 2005.


Importance of SFNB to Victorian barley crops

Spot form of net blotch has become the most prevalent foliar disease of barley crops in Victoria. It has consistently been identified in more than 90% of crops surveyed during 2003, 2004, 2005 and 2007 (see Table 1) and is expected to be common in 2008. SFNB is common in all barley growing regions of Victoria and has potential to cause yield loss if conditions are favourable. However, in general few crops are found to have SFNB levels likely to cause significant yield loss.

Other foliar diseases identified during surveys included scald (Rhyncosporium secalis), leaf rust (Puccinia hordei), powdery mildew (Erysiphe graminis) and net form of net blotch (Pyrenophora teres f. teres). Where present, scald was typically found at low levels but may potentially have caused yield loss in some paddocks. Leaf rust, powdery mildew and net form of net blotch were found at low levels and not likely to have been yield limiting.


Table 1. Prevalence (% paddocks infected) and maximum percentage leaf area affected by foliar diseases during four years of surveys in barley crops in Victoria


 
2003
(n=41)
2004
(n=43)
2005
(n=46)
2007
(n=61)
Disease
Prevalence
Max %LAA
Prevalence
Max %LAA
Prevalence
Max %LAA
Prevalence
Max %LAA
SFNB
92
19
98
7
96
18
95
33
Scald
41
19
32
8
26
20
13
2
Leaf rust
85
22
81
2
10
1
13
<1
Powdery mildew
26
15
28
1
4
<1
11
<1
NFNB
5
6
2
<1
0
0
0
0
n = number of paddocks surveyed


Control of SFNB using resistant varieties

Growing varieties with resistance is the most effective method of controlling disease. It is also the most economical for farmers as it reduces the need for foliar fungicide applications. Two field trials were sown in 2007 to determine the effectiveness of different sources of resistance in controlling SFNB. In this study, five varieties were grown and inoculated with infected barley stubble. The top four leaves were assessed for disease severity.

Significant reduction in SFNB severity was achieved at both sites when all four varieties possessing resistance were compared to the susceptible variety (Dash). A significant reduction in SFNB severity at the Wonwondah site (Table 2) indicates that even partial resistance can be adequate in controlling SFNB. Barque, which is a resistant variety provided the best control of SFNB with very little disease recorded.


Table 2. Disease severity (% leaf are affected) of SFNB on the top four leaves of five barley varieties at two locations in 2007


 
 
 
Horsham
Wonwondah
Variety
Rating
Resistance type
%LAA
GS59
%LAA
GS65
Dash
VS
-
1.54
11.9
Cowabbie
S
Seedling resistance
1.29
2.3
VB9104
NA
Adult resistance
1.13
2.3
Schooner
MS-S
Seedling resistance
0.71
2.4
Barque
MR
Complete resistance
0.22
0.6
P=
 
 
<0.001
<0.001
LSD(0.5%)
 
 
0.28
1.55

LSD(0.5%) 0.28 1.55
MR = moderately resistant, MS-S = moderately susceptible to susceptible, S = susceptible, VS = very susceptible, NA = rating not available


Control of SFNB using fungicide

Foliar fungicide application can be an effective method for controlling SFNB. In this study, a single application of propiconazole (250ml/ha) was applied at four different growth stages to determine which was most effective in controlling SFNB on the top four leaves.

A single foliar fungicide application provided significant control of SFNB symptoms on the top four leaves (Flag-4). However, in only one out of four years has significant benefit to grain yield from fungicide application been recorded (Table 3). Optimal timing of fungicide application to reduce SFNB severity was at growth stage Z31 in all four years. While a single application at growth stage Z39 also provided a significant reduction in SFNB severity. Fungicide application at growth stage Z59 did not reduce disease severity.


Table 3. Percentage leaf area affected (%LAA) by SFNB and subsequent grain yield in a very susceptible variety in response to fungicide application at four different growth stages during four years of study


 
Horsham 2003
Horsham 2004
Horsham 2005
Horsham 2007
Wonwondah 2007
Treatment
%LAA
Yield
%LAA
Yield
%LAA
Yield
%LAA
Yield
%LAA
Yield
GS25
-
-
-
-
13
3.5
2
3.3
21
4.9
GS31
6
4.8
16
1.8
10
3.5
1
3.2
16
5.2
GS39
7
4.7
25
2.0
-
-
1
3.2
16
5.1
GS59
16
4.8
38
1.8
12
3.2
2
3.1
21
5.2
Nil
15
4.8
34
1.7
15
3.2
2
3.1
21
5.2
LSD (0.5%)
na
ns
8.1
ns
na
0.24
0.437
ns
4
ns

* cvv. Gairdner was grown in 2003, 2004 and 2005. cvv. Dash was grown in 2007.

Conclusions

Spot form of net blotch has become the most common foliar disease of barley in Victoria. However, grain yield loss due to this disease is limited and in most cases its presence does not warrant control strategies (eg fungicide) to be put in place. To best control the disease and ensure that it doesn’t become a major problem to barley production in Victoria, resistant varieties are an effective control strategy. This study has shown that barley varieties with even partial resistance (seedling resistance) can provide control of SFNB. Further work is being done to identify sources of resistance to SFNB that can be incorporated into future barley varieties. Where susceptible varieties are grown and the risk of yield loss due to SFNB is high, foliar fungicides can be used and are best applied between growth stage Z31 and Z39. Economic benefit from using fungicides varies depending on seasonal conditions and barley market prices. More studies will be conducted in 2008 to further evaluate the importance of SFNB and effectiveness of control strategies.

Mark McLean ph 0353622111
Email mark.mclean@dpi.vic.gov.au