SUGGESTED FALL DORMANCY RANGE AND MINIMUM ALFALFA CULTIVAR PEST RESISTANCE RATINGS FOR SIX CALIFORNIA CLIMATE ZONES.  Growers selecting varieties from different regions should emphasize the pests that are most important for their area.
Rating Factor
  Fall Doramancy Spotted Alfalfa Aphid Pea Aphid Blue Alfalfa Aphid Phytopthora Root Rot Bacterial Wilt Fusarium Wilt Southern Anthracnose Stem Nematode Root Knot Nematode Verticillium Wilt  
Production Zone FD SAA PA BAA PRR BW FW An Stn RKN VW  
Intermountain 2--4 S R MR R R HR R R R R
Sacramento Valley 4--8 MR HR HR HR MR HR R R R R
San Joaquin Valley 7--9 R HR HR HR MR HR R HR HR R
Coastal 5--7 MR HR HR HR MR HR R HR HR R
High Desert 4--7 R R R R MR HR MR HR HR R
Low Desert 8--9 HR HR HR HR S HR HR R HR S  
NOTE: These pest resistance recommendations were originally developed by Dr. Vern Marble, Extension
Agronomist, UC Davis, based upon decades of experience with alfalfa varieties in various locations in California.
Zones correspond to the principle regions of alfalfa production in California. 
EXPLANATION OF PEST RESISTANCE.  Alfalfa varieties consist of a population of plants which have varying degrees of resistance to an insect or disease.  Since alfalfa fields can sustain considerable loss of individual plants without reducing productivity, alfalfa varieties with 51% or over are considered to be highly resistant, since resistant plants will make up for losses from other plants. 
Resistance Level Abbreviation       Percent resistance1    
Highly Resistant HR >51%
Resistant R 31-50%
Moderately Resistant MR 15-30%
Low Resistance LOW 6-14%
Susceptible S <5%
Tolerant T             (see definition)        
1 Percent of plants in a population resistant to a given pest
Definitions                        
I - Immune --  Not subject to attack for a specified pest.  Immunity is absolute, and seldom occurs in alfalfa.
R - Resistant --  The ability of plants to withstand pest attack. Resistance is not absolute but varies by degree.  Even highly reistant varieties will have some plants that are susceptible (see above percentages).  NOTE: Very high insect populations or very severe disease conditions can overwhelm pest resistance in alfalfa.
S - Susceptible --  Damage commonly occurs when in the presence of a specified pest. Inability of a variety to withstand adverse disease or insect conditions.
T -  Tolerant  --  Ability of plants to sustain yields when confronted with a pest attack or enviornmental condition (e.g. salt or grazing).  Tolerant varieties are affected by the condition, but still maintain yields at high levels relative to less tolerant varieties.