Planting/Multiplying CIMMYT Wheat Seed for Export

Seed Multiplication in Mexicali, Baja California, Mexico

The municipality of Mexicali, in northwestern Mexico, has officially been declared free from Karnal bunt (KB) according to the Diario Oficial de la Federación published on 15 December 1997, following the instructions given in federal Mexican law NOM-001-FITO-2001. The area is also recognized as KB-free by the North American Plant Protection Organization (NAPPO) and the European Plant Protection Organization (EPPO).

The CIMMYT Wheat Program uses fields in this certified zone (Mexicali) for multiplying seed intended for export from Mexico.

Seed Planting Procedures in Mexicali

Wheat, barley, and triticale germplasm to be included in an international nursery should be planted in plots assigned for that purpose at El Batán Experiment Station, an area free of Tilletia indica, causal agent of Karnal bunt.

During the crop cycle, plots will be sprayed with the systemic fungicide propiconazole (TILT) every 10 days, from spike emergence to the end of grainfilling, to protect the plots from aerial infections of T. indica.

After harvest, the seed is washed with a 1.2% sodium hypochlorite solution to destroy teliospores on the seed surface. Seed of each line is sampled by the SHL and filters from the seed wash examined under a stereo microscope (see Appendix). If no contamination is detected, the seed is treated with a mixture of carboxin + captan (Vitavax 300, 3 g/kg) and chlorothalonil (Daconil 2787, 2 g/kg). The seed is then shipped to Mexicali to be sown and multiplied.

During the crop cycle in Mexicali, the germplasm is constantly monitored to produce the highest quality seed. It is sprayed two to three times with propiconazole following the same method as before. All sowing and harvesting machinery in Mexicali remains there, i.e., it is never moved to non-certified areas where the disease might be present. Visitors to CIMMYT multiplication fields in Mexicali are required to bring clean clothes and shoes to wear and to wash their vehicles before arrival.

The germplasm is inspected at the end of the cycle, close to harvest time. Randomly sampled spikes are collected, individually threshed, and visually examined to detect potential seed health problems. The seed is then tested following procedures described in the Appendix. If the filter test results are negative, each line is individually harvested.

The harvested seed is placed in new bags that have been appropriately tagged. The bags and tags used at harvest must be shipped by air from KB-free areas to Mexicali. Harvested seed is appropriately packed and transported to CIMMYT headquarters in El Batan, State of Mexico, in a sealed truck that does not pass through KB-contaminated areas.

To avoid contamination, the truck is vacuumed and washed before being loaded. Before coming into the CIMMYT station at El Batan, the truck exterior is washed and vacuumed once again, to get rid of any contaminating pathogens, soil, or debris. The truck is then parked in a restricted area reserved exclusively for international nursery seed.

The inside of the storage area at El Batan is washed with a 1.2% sodium hypochlorite solution, and a sample of the wash water is taken to confirm that the area is free of any contaminants. All areas and all procedures are under zero tolerance for KB.

Lines received in the International Nurseries area can now be selected by CIMMYT for inclusion in an international trial. The selected lines are cleaned by sifting through screens to eliminate weed seed and large debris. Before international distribution, every line of every nursery is washed and sampled for the filter test using the following procedure:

The total weight of a line (if exceeding 1 kg) is divided into amounts of 800-1000 g and placed in perforated metal trays. Trays containing seed are then placed under high water pressure + 10 ml of Tween 20 for 3 minutes at 50-60 lb/in²; immersed for 3 minutes in 1.5% sodium hypochlorite solution; and rinsed for 2 minutes under high water pressure.

After washing, a seed sample (about 50 g) is taken from each tray and sent to the SHL. Depending on the amount of seed from the line, the sample is sub-sampled and one or more filter tests for the presence of T. indica and Tilletia spp. teliospores are carried out as described in the Appendix.

While the tests are being conducted, the remaining seed, after washing, is treated with a slurry containing chlorothalonil (Daconil 2787, 2 g/kg) and carboxin+captan (Vitavax 300, 3 g/kg), plus an adhesive (Pellistac). The seed is oven dried at 30 degrees C. If the filter tests results are positive for the presence of T. indica teliospores, the line is discarded, even if it has already been treated.

Washed, treated, and dried seed is placed in envelopes, and the nursery is assembled for shipping to CIMMYT cooperators. Seed selected by visitors during the Obregon cycle (see “Miscellaneous shipments” below) must follow this same procedure.

Miscellaneous Shipments

Occasional nurseries called “miscellaneous” or “special” nurseries that have not been multiplied in Mexicali are also send to collaborators. Any seed that was not multiplied in Mexicali merits special care for several reasons:

• Seed produced in El Batan or Toluca is of lower quality, mainly due to environmental conditions (high rainfall during the crop cycle) and higher disease incidence.
• The crop is not always sprayed with fungicide during the cycle as it is in Mexicali.
• Though El Batan and Toluca are KB-free areas, they have not been officially recognized as such.

Every line is visually inspected to determine the seed’s overall condition. When seed quality is very low (5-10% of the seed shows spots or other malformations), the breeder is strongly urged to select the seed before delivering it to the SHL and shipping it to collaborators.

Accurate laboratory examination is required for Karnal bunt and other diseases of quarantine concern. All lines must be delivered to the lab for inspection and the filter wash test. It is up to SHL personnel to decide whether to test every line or make up a composite sample for testing. Seed must be washed and chemically treated (as described above) before shipment, as for International Nursery distribution.

Detection of Other Seed-borne Pathogens of Wheat

Tests carried out by the SHL on wheat, triticale, and barley are given in Table 1.

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