Superior genetics: First line of defense
Much progress has been made by alfalfa breeders in the last 30 years in improving the genetic resistance of alfalfa varieties to diseases. Proper variety selection is the grower’s best line of defense against disease- incurred production losses. It is hard not to overemphasize this point.

In tough economic times growers can become fixated on the initial cost of the alfalfa seed, but often fail to realize that the variety choice they make influences the extent and severity of any future disease outbreak, and more importantly, the length of time that field will remain profitable.

Disease-induced yield and stand loss can cause the grower to prematurely take a field out of production. The resulting replanting costs are often significantly more than what he would have paid for an improved disease-resistant variety. It is estimated that even a half-ton per acre yield increase in a single year will more than offset the additional seed cost of a genetically superior variety.

Identifying healthy and diseased plants
One of the simplest tools a grower has at his disposal for gauging the health of an alfalfa field is his shovel. Dig some plants and examine the roots – a healthy alfalfa root should be relatively white inside. Any brown or red discoloration inside the root or crown is an indication that the root system is suffering from a disease infection (see Figure 1). The severity of the damage tends to get worse with age, and eventually results in plant death and stand loss.

If you are not sure if there is a disease problem, take the plant sample into your nearest extension agent and he/she can help you with the identification.

Cultural practices for disease management
Some alfalfa pests, like insects, can be sprayed with pesticides to prevent yield losses. However, with the exception of seed treatments for seedling damping-off diseases, none of the major alfalfa diseases can be economically controlled by any chemical treatment once a field is planted.

The remaining grower option is cultural practices, which usually consist of a two-pronged approach:

1. disease prevention

2. practices that limit disease severity

Preventing disease outbreaks
The grower’s first line of defense in preventing diseases is the variety. He should always plant certified disease-free seed, of a variety with high resistance or resistance for those diseases known to limit production in his region.

Once a field is planted, the following cultural practices are useful in disease control:

• Avoid spreading disease spores from old fields into newly planted fields by cutting newer fields prior to older fields to reduce disease infection.

• Before moving between fields, disinfect equipment by washing or spraying with a 10 percent bleach solution to prevent diseased soil or plant residue transfer.

• Reduce excess water in fields by tiling when necessary.

• In irrigated regions, use land preparation and irrigation practices that limit excess water conditions (i.e. land leveling, sprinkler versus flood irrigation). If using furrow or flood irrigation, shorten irrigation distances to avoid prolonged water-logged conditions.

• Avoid reuse of excess irrigation water from diseased fields.

• Minimize equipment traffic and excessive cultivation in established fields to reduce damaged or wounded plants that are more susceptible to disease infection.

Major alfalfa diseases and control measures
Beyond variety selection, field sanitation is the grower’s next-best disease control measure. Management practices preventing transfer of diseased plant material from older diseased fields into newly planted fields should be an integral part of every grower’s operation. The grower should also avoid any activity that wounds the plant’s crown or taproot, since it provides an entrance site for pathogens (i.e. excessive harrowing, cultivation, continuous grazing or wheel traffic on wet fields).

• Crown rot
Optimal disease conditions – Farming activities that wound the crown and taproot.

Symptoms – Stunting of plants; red to reddish-brown discoloration inside the root that becomes more severe with age of stand.

Control – Resistant varieties; minimize root and crown damage due to cultivation, grazing and wheel traffic.

• Phytophthora root rot and Aphanomyces
Optimal disease conditions – Occurs most often in soils with poor drainage or where water stands for an extended amount of time (more than 24 hours).

Symptoms – May be seen early as seedling damping-off in cool wet soils, or later in established plants as stunting or plant death in low areas of field where water stands. Taproots may be girdled and lateral roots may be brown in color. Top growth may be wilted due to poor water uptake from damaged roots.

Control – Resistant varieties; cultural practices that promote better drainage i.e. deep plowing, tiling, laser leveling and planting on beds. Use fungicide seed treatment to prevent seeding damping-off.

• Bacterial wilt
Optimal disease conditions – Most common in cold climates.

Symptoms – Stunting of plants; yellowish to brown discoloration inside the root that becomes more severe with age of stand.

Control – Resistant varieties. Cultural practices that limit crown/root damage resulting from excessive cultivation, grazing or repeated wheel traffic damage.

• Fusarium wilt
Optimal disease conditions – Most common in warm climates.

Symptoms – Stunting of plants; red to reddish brown discoloration inside the root that becomes more severe with age of stand.

Control – Resistant varieties; cultural practices that limit crown/root damage resulting from cultivation, grazing or repeated wheel traffic damage.

• Anthracnose
Optimal disease conditions – Occurs in spring or fall and spreads rapidly under warm wet conditions from spores produced on lower stems of infected plants.

Symptoms – Individual straw- colored stems on plants that display a curved top “shepherd’s crook.” Diamond-shaped lesions will occur on lower part of the stem.

Control – Resistant varieties; growers should avoid spreading disease spores from plant debris on harvest equipment to uninfected fields.

• Verticillium wilt
Optimal disease conditions – The pathogen can be spread by dry or fresh plant material on harvest equipment. Cutterbar blades of mowing equipment are extremely effective in spreading the pathogen.

Symptoms – Stunting of plants; Yellow ‘V’ shape at the tip of leaves. Leaves may curl along midrib and turn a pinkish color. Stems will remain green after leaves die.

Control – Resistant varieties; clean farm equipment between fields, and mow younger fields before older fields to prevent spore transfer on mower blades.

Summary
A grower’s first line of defense against alfalfa diseases should always be the variety he plants. A disease- resistant variety’s built-in genetic protection is the best insurance policy a grower can have against yield losses due to diseases. That resistance, combined with proper cultural practices, is the grower’s best defense in minimizing disease-incurred production losses.  FG