Maize Genetics Cooperation Newsletter vol 84 2010

Please Note: Notes submitted to the Maize Genetics Cooperation Newsletter may be cited only with consent of authors.

 

 

Ottawa, Ontario, Canada

Eastern Cereal and Oilseed Research Centre

 

Expression of different Ht genes to Northern Corn Leaf Blight in Ottawa, Ontario

Zhu, X., L. M. Reid, T. Woldmariam, C.Voloaca, and J. Wu

 

Twenty-nine inbred lines with different Ht resistance genes for northern corn leaf blight [ Exserohilum turcicum (Pass.) K.J. Leonard & E.G. Suggs (teleomorph = Setosphaeria turcica (Luttrell) K. J. Leonard & E.G. Suggs; syn. = Helminthosporium turcicum Pass.]  and four susceptible inbred lines (A619, Pa91, CO388, and CO442) were evaluated for the expression of resistance in our leaf blight breeding nursery at Agriculture and Agri-Food Canada (AAFC), Eastern Cereal and Oilseed Research Centre (ECORC), Ottawa, Ontario in 2006 and 2007.  Each genotype was planted in a five-row plot with single row plots of 15 plants.  All plants were inoculated twice with 0.1 g of ground diseased leaf powder collected from the previous field season.  The powder was placed into the whorl of each plant using a Bazooka (Sistrunk Inoculators, Starkville, MS 39759, USA) at 6-8 leaf-stages and again at the 10-12 leaf–stages.  If there was no rain after inoculation, the plots were irrigated for 10-15 minutes every day to provide suitable environment for disease to develop.

Specific resistance were observed and recorded first at 15-20 days after the first inoculation (about 12 leaf stage). Five specific resistances were observed, classified as: highly resistant (HR), resistant lesion (R), moderately resistant lesion (MR), moderately susceptible lesion (MS), and susceptible lesion (S).  For HR types, there were no typical lesions and the infection point changed color from yellow to brown, sometimes to purple, and had only limited extension.  The R types have stripes or narrow elliptical greenish-yellow lesions.  MR types have narrow, long, elliptical gray lesions with greenish-yellow or purple margins. MS lesions are long, elliptical, and gray lesion with greenish-yellow or purple margins. The S lesions are long, elliptical, and gray or tan in color. HR, R, and MR plants were selected to self for further resistance breeding to northern leaf blight.

At the soft dough stage (about 3 weeks after silk emergence), plants were observed the specific resistance second time, its classification as above, whereas general resistance were recorded.  Based on disease severity, the disease rating scale for general resistance ranges from 1 to 7 where: 1= no symptoms; 2= < 1% of the leaves are symptomatic; 3= 1- 10% of the leaves are symptomatic; 4= 11- 25% of the leaves are symptomatic; 5= > 50% of the lower leaves and   < 25% of the mid and upper leaves are symptomatic; 6= lower leaves are dead, > 50% of the mid leaves and  < 25% of the upper leaves are symptomatic; 7= plant is dead.  The mid leaves refer to the four leaves near the primary ear. Hand-pollinated plants with HR, R, or MR, and disease rating 5 were selected continued breeding. Plants with MS or S lesions, or disease rating 6, were discarded. 

The screening results of inbred lines in 2006 and 2007 were shown in Table 1.  Based on these results shown in the table, we made the following conclusions on the expression of the Ht genes in Ottawa:

1.                        Three of the initial sources of blight resistant inbreds [A509N, A553N (orange), and A553N (red)] did not have uniform (pure) resistance since HR or R and S lesions were observed in the 2006 screening. However, self pollination of the HR plants resulted in all HR plants when screened the following year.

2.                        We observed distinct differences in the expression of the Ht genes at early stages (about 12 leaf stage) of plant growth and as expected, symptoms in susceptible genotypes increased as the plants matured. Some specific observations were:

a.       Lesions on Ht and Ht1 plants were very similar. All Ht and Ht1 inbreds were susceptible in Ottawa, Ontario, and are thus not suitable sources of resistance for our region.

b.     All Ht2 and Ht3 inbred lines had similar MR type resistance in 2006 and 2007. They all had long lesions with yellow or purple margins.  During the later stages of plant development, some of these lesions became larger and developed a gray center thus becoming MS type lesions.

c.       All Htm1 and HtN inbred lines had HR or R type resistance. As the plant developed these either did not change or became slightly larger in size.


Table 1. Northern corn leaf blight screening results of inbred lines with different resistance in 2006 and 2007 in Ottawa, Ontario, Canada

 

 

 

2006

2007

Source

 

Name

Ht gene

Specific

Resistance

General Resistance

Specific

Resistance

General Resistance

Ames23458

A619HT

Ht1?

S

5

S

7

Ames23468

A632HT

Ht1?

S

5

S

6

Ames27065

B73Htrhm

Ht1?

S

5

S

6

PI601079

LH123Ht

Ht1?

S

5

S

6

Ames27138

N28Ht

Ht1?

S

5

S

7

Ames25219

A619Ht1

Ht1

S

5

MR

6

Ames25372

Pa91Ht1

Ht1

S

5

S

6

Ames25220

A619Ht2

Ht2

MR

4

MR

6

Ames25221

A619Ht3

Ht2

MR

4

MR

5

Ames25373

Pa91Ht2

Ht2

MR

4

MR

5

Ames25374

Pa91Ht3

Ht3

MR

4

MR

5

Cornell University

73353

Htm1

R

3

R

3

PI550496

H102

Htm1

HR/R

2

HR/R

3

PI406112

A214N

HtN

HR/R

2

HR/R

3

PI406118

A509N

HtN

95.2HR/R:4.8S

2 or 4

HR/R

3

PI406119

A553N (Orange)

HtN

79.7HR/R:20.3S

2 or 4

HR/R

3

PI406120

A553N (Red)

HtN

83.3HR/R:16.7S

2 0r 4

HR/R

3

Ames23469

A632HtN

HtN

HR/R

2

HR/R

3

PI406126

A661N

HtN

HR/R

2

HR/R

3

PI587142

A679

?

--

--

R

5

PI584529

B100

 ?

--

--

R

3

PI594045

B102

?

--

--

R

3

PI608777

B85

?

--

--

R

3

PI539870

B92

?

--

--

R

3

PI539871

B93

?

--

--

HR/R

2

PI561565

B95

?

--

--

MR

6

AAFC

CO428

?

R/MR

3

R/MR

3

PI587129

H99

?

--

--

R

4

Ames27141

NC290A

?

--

--

R

4

PI587139

A619

N/A

S

7

S

7

PI587174

Pa91

N/A

S

7

S

7

AAFC

CO388

N/A

S

7

S

7

AAFC

CO442

N/A

S

7

S

7

Total

 

 

 

24

 

33

Name of organization from which the inbred was obtained or United States Department of Agriculture NC-7 code.

Ht resistant gene according to inbred description or pedigree

Resistance of these inbred lines was not uniform upon first screening in 2006. The numbers for specific resistance lesion types refer to the percentage observed for each type.

Specific Resistance: HR = highly resistant; R = resistant lesion(s); MR = moderately resistant lesions; MS = moderately susceptible lesions; and S = susceptible lesions.

General Resistance: 1= no symptoms; 2= < 1% of the leaves are symptomatic; 3= 1- 10% of the leaves are symptomatic; 4= 11- 25% of the leaves are symptomatic; 5= > 50% of the lower leaves and   < 25% of the mid and upper leaves are symptomatic; 6= lower leaves are dead, > 50% of the mid leaves and < 25% of the upper leaves are symptomatic; 7= plant is dead.