Alfalfa Disease Resistance Index (DRI) How it is Calculated and Used in Variety Selection
By Don Miller, Alforex Seeds
Over the last half century alfalfa breeders have made tremendous advances in improving alfalfa varieties. In the last 5 years there have been 325 alfalfa cultivars approved by the National Alfalfa Variety Review Board. Of those,181 are currently listed on the national variety list as being actively marketed. Tools like the Alfalfa Disease Resistance Index (DRI) make rating alfalfa easier.
With so many varietal options, producers are faced with the task of narrowing down the list of potential varieties for their farm to those with the most genetic advantage for profitability. Using the Fall Dormancy and Winter Survival Ratings can narrow the list significantly to those varieties adapted to a specific production area’s climate and environment. Further selection can be made based on desirable traits such as yield and/or forage quality. However the final determination in varietal selection is often made using a varieties disease resistance package, since diseases can greatly influence long term persistence and productivity. The Disease Resistance Index (DRI) is a simple tool producers are using to identify varieties with an optimal disease package for their local conditions.
The DRI was developed as a quick calculation that provided producers with a single number that could be used to quantify each varieties level of resistance and its combined resistance to the major alfalfa diseases. Initially 5 alfalfa diseases were used in the calculation, Bacterial wilt, Fusarium wilt, Verticillium wilt, Anthracnose, and Phytophthora Root Rot. A point value was given for the level of resistance to each disease, with the most points given for high resistance; Susceptible (S) =1, Low Resistance (LR) =2, Moderate Resistance (MR)=3, Resistant (R) =4, and High Resistance (HR) = 5. The point score for all five diseases was added up to determine the Varieties DRI score. Varieties with high resistance (HR) to all five diseases had a DRI score of five HR’s for five diseases (5 x 5) for a total of 25 out of a possible 25. Varieties with four “HR’s” (4 x 5=20 points) and one “R” (1 x 4 = 4 points), had a total of 20 + 4 =24 out of 25.
In the 1980’s Aphanomyces Root Rot (Race 1) was added to the list, and the DRI calculation now included 6 disease , and the best DRI score went from a possible 25 to 30. Most recently Aphanomyces Root Rot (Race 2) has also been added and now we have a new disease score using 7 diseases for a DRI with a possible score of 35 if a varieties had high resistance(HR) to all 7 diseases.
Example of Disease Resistance Index (DRI) Calculations
How Many Diseases Should Make Up The DRI Calculation? Are DRI Numbers Using Unofficial Races Of Aphanomyces and Anthracnose Appropriate At This Time?
The Alfalfa Diseases Rating Index number has been very useful in helping producers to quantify an alfalfa varieties’ overall level of disease resistance. However we should be careful to not abuse the system and add disease races to the DRI calculation that are not currently officially recognized by the National Alfalfa Variety Review Board(NAVRB). A prime example are DRI claims using Aphanomyces (Race 3) and Anthracnose (Race 5) in an expanded DRI calculation.
The NAVRB currently only recognizes 2 major races of Aphanomyces, Race 1 & 2. Both have been identified in a wide range of major production areas with Race 2 being considered the most detrimental to alfalfa production. USDA-ARS researchers have identified as many as 20 other isolates of Aphanomyces, however at this time none have been officially recognized and characterized with official testing protocols using susceptible and resistant checks by the NAVRB. The same situation exists for a new disease isolate of Anthracnose labeled as (Race 5).
New alfalfa varieties and their resistance levels to Aphanomyces Race 1 and 2 and Anthracnose Race 1 are verified by the NAVRB with standardized replicated tests using industry approved checks. Without standard checks, a new varieties level of resistance to new disease isolates can’t be determined. Therefore it is impossible to determine if a variety has “HR” ,“R”, “MR”,”LR” or “S” for the new disease isolate. Without those resistance levels, an accurate DRI calculation can’t be made.
A case might be made for adding these new disease isolates to the NAVRB official list, and it is being considered by the alfalfa marketers and researchers. However the discussion has not progressed to the point where there are official standard check varieties and testing protocols for the new disease isolates.
Although only two races of Aphanomyces and one of Anthracnose are officially recognized, undoubtedly researchers are currently developing genetic resistance to these and additional new races. Plant Breeders often screen for disease resistance using a broad mixture of pathogens for plant selection. These mixtures also include regional isolates that may not be considered significant enough to be identified as an official race. Using multiple isolates in the variety development provides a broad spectrum of disease resistance to each disease, even though the exact level of resistance to each regional isolate is not known.
In conclusion, DRI varietal scores can be very useful in determining a varieties combined disease resistance. However calculations using unofficial “races” of a disease to inflate the DRI number, is premature and should be avoided, until the National Alfalfa Variety Review Board agrees on an official testing protocol & check varieties that allow an accurate determination of the levels of resistance (S to HR) needed for a DRI calculation. Only then can the DRI be expanded to include a new disease race with accuracy. Buyers should check the NAFA “Winter Survival, Fall Dormancy & Pest Resistance Ratings for Alfalfa Varieties” for those official disease resistance ratings, and use those to calculate an accurate DRI.