Alfalfa diseases in north-central Mexico

Y.I. Chew M., J.J. Márquez O., and J. Santamaría C.

Instituto Nacional de Investigaciones Forestales Agrícolas y Pecuarias.

C.E. La Laguna. Apdo. Postal 247, CP 27000 Torreón, Coah. Mexico.

Alfalfa is the most important forage crop (35,000 ha) at La Laguna, the largest dairy production area in north-central Mexico. However, despite that some plant diseases are a major cause for low alfalfa production in certain areas of La Laguna, there is a lack of general information of how plant diseases affect alfalfa production in this region. Therefore, our objective was to identify the most important diseases, to determine their time of appearance throughout the year, and their impact on forage production in the area.

We sampled 20 farms of La Laguna in March, June, August, and November of 1995 and 1996. Thirty whole plants were dug from the field at each farm. In March and November of both years we took soil samples (0-50 cm depth) to determine nematode incidence. In March and November of 1997 samplings were made at six farms to estimate yield losses caused by crown rot. Plants from each ranch were classified based on degree of damage (severity) caused by the crown rot complex (0=no damage; 1=1 to 25% damage; 2=26 to 50% damage, and 3=>50%). We measured fresh and dry weight of twenty-five plants from each rank of the scale. Forage losses at one, two, and three years of stand age were estimated using yield:severity ratios and percentages of damaged plants.

The most important diseases found in the area were crown rot (Fusarium spp, Rhizoctonia solani, Phoma sp, and Colletotrichum sp), anthracnose (Colletotrichum sp), Texas root rot (Phymatotrichum omnivorum), mildew (Peronospora trifoliorum), and rust (Uromyces striatus).

Crown rot incidence was almost 100% for both years, starting in the summer. Anthracnose was observed in wilted stems, but also associated to crown rot (85% in 1995 and 70% in 1996). Stand age was positively correlated to damage caused by crown rot and anthracnose. Texas root rot had an incidence of 55% in 1996 and 50% in 1996, starting in May, when temperatures increased.

Mildew was found in March 1995 (50%) and March and November 1996 (20%). Rust showed up in August 1995 (30%) and August and November 1996 (20%). Differences among years were caused by increased moisture conditions in October 1996.

Nematodes found were Pratylenchus sp, Helicotylenchus sp, Tylenchorhynchus sp, and Xiphinema sp, but they were not considered a problem.

We estimated that alfalfa crown rot reduces dry matter forage yield 11.6, 28.0, and 33.33% in the first, second, and third year of stand age, respectively.