Seed-borne diseases to consider before using saved seed
Wheat harvest is winding down in most of Missouri and some producers may be thinking of saving seed to use for planting this fall. This is a year when it would be wise to consider the possibility of seed-borne diseases and how they might impact seed quality and stand establishment. The Septoria/Stagonospora complex and bacterial leaf streak/black chaff both came on late in the season causing some head discoloration. After head emergence there were more questions than usual about loose smut. Finally Fusarium head blight or scab was fairly widespread in Missouri this season. All of these pathogens can be carried on or in the wheat seed, reducing germination, causing seedling blights or causing disease problems the next season. If any of these diseases were present at significant levels in a field, it would be best not to use seed from that field for planting this fall. If wheat is going to be saved for seed, this is certainly a year to pay careful attention to the quality of seed being saved.
Lesions of Septoria leaf blotch begin as light yellow flecks or streaks. These flecks expand into yellow to reddish-brown, irregularly shaped blotches. Dark brown specks (fruiting bodies or pycnida of the causal fungus, Septoria tritici) may be scattered within the centers of mature lesions. Lesions may coalesce killing larger areas of leaf tissue. Stagonospora glume blotch (formerly called Septoria glume blotch) may also begin as light yellow flecks or streaks on leaves. The lesions also turn yellow to reddish-brown but usually have a more oval to lens shaped appearance than those of Septoria leaf blotch. Again, the dark brown specks or fungal fruiting bodies of the causal fungus Stagonospora nodorum may be evident within the lesions. Symptoms of Stagonospora glume blotch are more common on heads than foliage of wheat. Infected heads will have dark blotches on the glumes. Stagonospora is more likely to be seed-borne than is Septoria. Seed lots infected with Stagonospora may have a greater risk for stand establishment problems as the fungus can cause seedling blight under a range of soil temperatures.
Bacterial streak and black chaff are names for the same bacterial disease which produces symptoms on both leaves and heads. Water-soaked lesions may develop on young leaves. These develop into reddish brown to brownish black streaks on the leaves. Glumes and awns show brown-black blotches or streaks. Black chaff may be confused with glume blotch. Symptoms may not be evident on individual kernels but the bacterial pathogen can be seedborne. Since seed treatment fungicides are not effective against this bacterial pathogen, seed from fields which had bacterial streak and black chaff should not be used for planting.
This season there were more phone calls than normal related to loose smut of wheat. Loose smut would have been quite easy to see in the field at heading and early grain fill stages of growth. The kernels on infected heads are replaced with masses of powdery black spores. So the heads have a very obvious, black, powdery appearance. These spores are eventually dislodged by wind and rain, so later in the season the smutted stems are less evident and only the bare rachis will be left. Spores produced on smutted heads are wind carried to adjacent plants in the field and infecting through the flowers. The fungus that causes loose smut survives within the embryo of wheat seeds. Infected seed does not show visible symptoms and will germinate normally. However, if infected seed is planted, the plants growing from those seeds will be infected and develop smutted heads the next season. If seed from a field that has a “small” amount of smut in one season is used for seed, the field planted with that seed may have a substantially higher level of smut. Loose smut is best controlled by planting either disease-free seed or using the proper rate of a systemic fungicide seed treatment labeled for the control of loose smut.
Fusarium head blight or scab infection may result in shriveled and shrunken kernels, lightweight bleached or tombstone kernels or kernels that have a pinkish cast or discoloration. Lots with high levels of scab may have lower germination rates. The fungus that causes scab can also cause a seedling blight of wheat. If scab infected seed is used for planting, seedling blights and stand establishment problems may occur. Management of Fusarium seedling blight is through the planting of disease-free seed or a combination of thoroughly cleaning the seed lot, having a germination test run, adjusting the seeding rate to compensate for germination rate and using a fungicide seed treatment effective against seed-borne Fusarium or scab.
If any of the diseases covered in this article were present in a field this past season, it would be prudent not to use seed from that field for planting this fall. If seed must be used for planting is should be thoroughly cleaned to remove all shriveled, shrunken and lightweight kernels. A germination test would be recommended. For Stagonospora, loose smut and Fusarium head blight a fungicide seed treatment may be necessary. A number of fungicides are labeled for use as seed treatment fungicides on winter wheat. These seed treatment fungicides protect germinating seed and young seedlings from seed-borne and soil-borne pathogens. Seed treatment fungicides will not improve germination of seed that has been injured by environmental factors and will not resurrect dead seed. A correct assessment of the cause of poor seed quality or poor germination rates is the first step in deciding if a seed treatment fungicide is necessary.
Fungicide seed treatments for winter wheat are included in the 2013 Pest Management Guide: Corn, Grain Sorghum, Soybean and Winter Wheat, Extension Publication M171. Printed copies of this bulletin are available from the Extension Publications Distribution Center, 2800 Maguire Blvd., Columbia, MO, 573-882-7216 or on-line at http://extension.missouri.edu/p/M171 through MU’s Extension Publications.