ADVANCES IN BREEDING ALFALFA FOR RESISTANCE TO PLH

 

Mark McCaslin and Dave Whalen

Forage Genetics

 

Potato Leafhopper (PLH) resistant alfalfa varieties have been marketed since 1997.  The performance results of the first PLH resistant varieties from various public and private trials can be summarized as follows:  When PLH were above economic threshold, PLH resistant varieties showed a 5-15% yield advantage over commercial checks.  When PLH were below economic threshold (i.e low insect pressure or the test is sprayed for PLH control), PLH varieties show a 5-10% yield disadvantage over commercial checks.  Each of the years 1997-99 had above average PLH pressure over most of the Midwest.  Information from research trials and grower feedback strongly indicated that the level of resistance in first generation PLH varieties (25-35% resistance) did not provide adequate control when PLH populations were high. As the second and third generation varieties came on the market, both resistance levels and agronomic performance were improved. While these new varieties were superior to the first generation PLH resistant varieties, the majority of these products still did not perform as well as the best conventional varieties in the absense of leafhopper pressure.

 

Our breeding effort for PLH resistance has focused on two areas: increased resistance to the insect and improved agronomic performance.  A successful commercial product, with consistent performance, will need both improvements.  This breeding goal is complicated by the fact that wild relatives have contributed both the genetic source of resistance and the genes contributing to “yield drag”.  

 

Increasing levels of resistance has been pretty straightforward.  A combination of growth chamber and field screening has resulted in 2-3 fold improvements in PLH resistance levels.  1999 performance results of our newest experimentals from 1998 seeded trials suggest that concurrent selection for late fall dormancy, fast recovery after cutting and improved vegetative vigor have now provided a key breakthrough in agronomic performance.  The table below shows performance of two of these improved PLH experimentals compared with commercial checks.

 

 

Variety

FD

%R

Plh

Forage yield DM T/A

Mean % checks

8IN3

8IA3

8WI6

8WI3

4M17

3.5

75

3.50

6.50

7.42

8.43

124

4M25

3.6

73

3.94

6.56

7.25

8.29

125

Magnum IV

4.0

0

1.92

5.26

5.95

7.56

99

5347LH

3.0

25

2.43

5.53

6.04

7.28

102

5454

4.0

0

2.02

5.53

5.76

7.43

99

 

 

The trials were seeded in August 1998.  The test locations, in decreasing order of PLH pressure, are Lafayette, IN (8IN3), Ames, IA (8IA3), Madison, WI (8WI6) and West Salem, WI (8WI3).  At the Iowa and Wisconsin locations, the best PLH varieties had higher yields than the commercial checks at each of four cuttings, including the 1st cut when PLH numbers are very low (i.e. before significant PLH migration occurs).  Although these results are from first harvest year, they agree with observations from 2-3 year old breeding nurseries, and suggest a breakthrough in improved agronomic performance.

 

Current PLH resistant varieties (PLH1998) are primarily targeted at regions where PLH problems are common, and chemical control is not used.  Although this is a significant market niche, it represents a fraction of what the potential area of adaptation for new products (PLH2001) with high PLH resistance and with yield > commercial check cultivars in a no/low PLH environment.  The first of these improved products should come on the market in the spring of 2001 after additional performance tests are completed. The map below shows current and potential niches for PLH resistant alfalfa varieties.