In the Northern Plains, there are several economically important diseases that are seed-borne. Effective prevention and control of these diseases often begins with reducing the introduction of the inoculum in the field. Seed health testing is important to determine whether these pathogens are present in seed and at what levels. In some crops, seed certification depends on a satisfactory disease test result.
Barley loose smut is a disease that infects barley during the flowering stages of development. This disease is seed transmitted and can cause significant yield losses in the field. Today, the use of fungicides and resistant cultivars has resulted in loose smut becoming less prevalent and damaging than in the past. It is important to note that this disease is present during every growing season to various degrees and has the potential to cause yield loss if control measures are not followed. Wet growing seasons or wet areas may have higher levels of seed infection than dryer areas or dry years.
Symptoms
Barley loose smut is a fungal disease caused by the organism Ustilago nuda. Loose smut only survives to the next growing season as dormant mycelium inside barley seed embryos. Once infected seed are planted, the fungus begins to grow and infects head tissue. The infected head becomes a source of spores to infect other plants during flowering.
Control
The Seed Department requires all certified barley seed in North Dakota to be tested for loose smut. There are no set limits on seed infection levels, however, at levels of 2% seed infection or higher, the department recommends application of a seed treatment before planting. This disease can be controlled effectively by planting resistant cultivars or seed-applied fungicides.
Testing
The embryo count method is used where the number of infected embryos in 500 seed are determined. Results of this test are reported as percent infected seed of 500 seed tested. This test is typically completed in two to three days.
Bean anthracnose is a potentially devastating disease that remains a concern for edible bean producers in North Dakota. The main difficulty with anthracnose, in addition to seed-borne transmission, is that under low infection levels it may be difficult to detect in the field so it can be easily overlooked by growers. In fact, symptoms of anthracnose on bean pods can look similar to those of bean bacterial blight. Low infection levels in the seed have the potential to cause a severe outbreak of the disease the following year under optimal conditions. Thus, testing for anthracnose is extremely important and required for seed certification.
Symptoms
Field symptoms of anthracnose appear as small, angular brick red to purple-brown lesions on the bottom of leaves. Later, these lesions become darker, extend to the upper leaf surface, and proceed along veins. Pod lesions are sunken, circular in shape with brown to black coloring that have a dark margin surrounding the lesion. There is typically a thin zone of red tissue around the lesion. On the lesion surface, tan spores dry into dark granular masses. Lesions on seed can be similar in appearance to pod lesions. Bean anthracnose is easily spread by infected seed, splashing rain and wind blowing infected crop residue from field to field. This disease can result in significant yield losses.
Control
The North Dakota State Seed Department has taken measures to ensure certified bean seed produced in North Dakota is free from this serious pathogen. In 2002, North Dakota imposed mandatory anthracnose testing on all certified seed grown in the state. Additionally, service testing is being promoted and utilized by the seed industry on seed lots offered for sale in North Dakota. Growers should be alert as to the origin of the seed they are purchasing. Seed from known infected areas should be avoided. Good management practices are important for prevention of anthracnose. Growers should avoid planting bin run seed. The use of certified seed that has been field inspected and lab tested for anthracnose is recommended.
Testing
The anthracnose test is required for certification of dry edible bean in North Dakota. We strongly encourage all edible bean seed producers to test each field separately for anthracnose. Testing each field separately is important if seed from a number of different fields is commingled into a single seed lot. If seed from one field infected by anthracnose were commingled with seed from other fields that did not have anthracnose, the entire seed lot would then become contaminated.
The Seed Department currently conducts a 1,000 seed grow-out test for bean anthracnose that requires a minimum of 14 days to complete. Seedlings are evaluated for the presence of the anthracnose causing fungus Colletotrichum lindemuthianum. A positive test result indicates the sample is contaminated, resulting in failure. Growers should be aware of the sample size testing labs use to conduct an anthracnose test on edible bean. Sample sizes of 200-400 seed may not be large enough to detect the presence of anthracnose. Growers must submit a two to three pound sample for this test. The Bacterial Blight (Dome) test can be performed at the same time as an Anthracnose test. Submit at least three pounds of seed for a Dome test and Anthracnose test. For these tests, a good representative sample of the seed lot is important, as very low levels of anthracnose infection are a serious problem. This test is a pass or fail test and any positive test result will result in a failed test.
Additional Resources
Bacterial blight has traditionally been the greatest factor contributing to bean fields failing field inspections. There are two primary types of bacterial blight found on dry edible beans in North Dakota, halo blight and common blight. Each is caused by a specific bacterial pathogen. Since bacterial blight is seed-borne, all bean seed eligible for certification in North Dakota must be tested for the presence of the pathogen. North Dakota Field Seed Certification Standards specifically state that growers are required to submit seed samples of the harvested seed of each field or seed lot of dry field beans for bacterial blight testing.
Symptoms
Field symptoms of common bean bacterial blight include water-soaked lesions on leaves and pods that later develop into larger brown necrotic areas on leaf surfaces and into circular brownish lesions on pods. Severely infected leaves may have a “burnt” appearance. Infected seed may be shrunken, shriveled, and discolored. Halo blight symptoms on leaves appear as a yellow ring around each lesion. Pods develop water-soaked brown lesions similar to common bacterial blight. Halo blight also causes seed to become shriveled and discolored.
Control
Effective control measures include planting disease-free seed, planting resistant cultivars and avoiding fields with a history of the disease.
Testing
The bacterial blight test, known as the dome test, has been used by the department for a number of years to detect bacterial blight in dry edible beans. The dome test measures the symptoms of blight in the form of water soaked lesions on the undersides of primary bean leaves. These results are calculated on the basis of the number of lesions observed on the primary leaves of 13-day-old plants. The average lesion number is determined on 30 plants and samples are typically run in duplicate. The average lesion number corresponds to the average area of the leaf surface covered by lesions. The dome test provides a blight rating score for each sample, permitting an easy comparison of seed lots. Lower dome scores equate to lower amounts of blight in the seed. Dome scores of four or less are considered acceptable for certification.
Additional Resources
Blackleg is a serious fungal disease of canola caused by Leptosphaeria maculans. This disease occurs in both mild and virulent forms. Both types are found in North Dakota and many parts of Canada. The virulent strain is economically important as it may cause premature seedling death in the field or result in canola plants that lodge easier. This disease is seed transmitted.
Symptoms
Symptoms appear as tan or brown colored leaf spots on leaf surfaces. Stem lesions can also form and are gray or black in color. Pods and seed may also become infected. Infected seed may appear gray and have a shriveled appearance. Infected pods have a tendency to split open, resulting in seed loss.
Control
Effective control measures for Blackleg in canola include good management practices, planting resistant hybrids and use of disease-free seed.
Testing
The Seed Department offers testing for the virulent strain of Blackleg. An agar test is performed using 1,000 seed. This test is typically completed in seven days and results are reported as a percentage of infected seed.
The disease known as Ascochyta blight can be a serious problem in pulse crops such as chickpea, lentil, and field pea. Ascochyta blight is seed-borne and can also be spread by infected plant residue blown into a field. Ascochyta blight is crop specific, meaning that lentil Ascochyta blight will only infect lentil and not chickpea, field pea, or other pulse crops. The same is true for chickpea and field pea Ascochyta blight. There are similarities in the symptoms of Ascochyta blight on pulse crops, but controlling this disease varies among crop types.
Lentil Ascochyta
Symptoms of Ascochyta on lentils can occur on leaves, pods, and stems as white to tan spots with a darker outside margin. The centers of these lesions can be speckled with the black fruiting bodies which contain spores of the fungus. Infected seed can be discolored with a brownish color. Moist weather is conducive to the development and spread of the disease. Infection levels of 1% or greater are considered high. Ascochyta can cause yield losses of 30-50% in susceptible varieties.
Control
Effective control measures include crop rotation and application of fungicides and resistant varieties.
Chickpea Ascochyta
This disease can be devastating in chickpea as it can spread very quickly and can cause significant yield losses. All above ground portions of chickpea plants are susceptible and relatively low seed infection levels can cause serious problems in the field. In fact, seed infection levels of 0.3% or higher are considered high. In cases such as this, it would be best to look for a different seed source. Symptoms on chickpea include the development of dark, sunken lesions that contain rings on the outer margin. Spores are often present in these lesions. Cool, moist weather conditions favor development of this disease. However, spore production can occur under relatively dry conditions providing inoculum for secondary infections.
Control
The best control measures for chickpea are planting disease-free seed, resistant cultivars and crop rotation to non-host crops.
Field Pea Ascochyta
Field pea can be infected by more than one species of the fungus. All above ground portions of pea plants are susceptible to this disease. Symptoms on peas include the development of purplish black to brown spots or lesions on stems, leaves, and pods. Black spore producing structures may form on these lesions. Pod lesions may become sunken. The fungus can over-winter in seed, infected crop residue, and in the soil. Seed infection levels of 5% or greater are considered high.
Control
The best control measures are planting disease-free seed and crop rotation to non-host crops.
Testing
The Seed Department currently conducts seed tests for Ascochyta in lentil, field pea, and chickpea. The Ascochyta test is required for certification of chickpea and lentil seed. There is no pass/fail standard but the test results must be on the label. Results of this test are reported as percentage of infected seed of 500 seed tested. This test is typically completed in 8-10 days and requires varying amounts of seed depending upon the crop. Small seeded types such as lentil require a minimum of ½ lb. of seed while larger seeded types such as chickpea or field pea require a minimum of 1 lb. of seed. Chickpea samples can also be tested for Ascochyta using 1,000 seed if required.