49). T h e mycelium then follows closely the growth of the plant and grows best just behind the growing point, while the hyphae in the tis-sues of the lower stem atrophy and frequently disappear. Whe n the plant forms the head, and even before it e m e r g e s, the mycelium in-vades all the young spikelets, where it grows intracellularly and de-stroys most of the tissues of the spike, except the rachis. By this time, most of the infected plants are slightly taller than most healthy plants, probably d ue to stimulatory action of the pathogen. T h e mycelium in the infected kernels is soon transformed into teliospores which are contained only by a delicate outer m e m b r a ne of host tissue. T h e membranes burst open soon after maturation of the teliospores and the spores are released and blown off by air currents to nearby healthy plants. T h e spore release coincides with the o p e n i ng of the flowers of healthy plants. Teliospores landing on flowers germinate through formation of a basidium on which the haploid hyphae are produced.
After fusion of the sexually compatible haploid hyphae, the resulting dikaryotic mycelium penetrates the flower through the stigma or through the young ovary walls and b e c o m es established in the peri-carp, integuments, etc., and in the tissues of the embryo before the kernels b e c o me mature. T h e mycelium then b e c o m es inactive and remains dormant, primarily in the scutellum, until the infected kernel germinates.
Control
T h e pathogen of loose smut of barley and wheat causes systemic infection and overwinters inside the s e ed so that no surface applica-tions of fungicides on the plant or kernels can reach and control the fungus directly. Experimental control of loose smut of barley has b e e n obtained, however, by treating infected barley s e e ds with derivatives of 1,4-oxanthiin before planting. T h e se chemicals s e em to b e ab-sorbed and act systemically in the s e ed or in the growing plant.
Although some barley and wheat varieties are quite resistant to loose smut, most of the commercial varieties are very susceptible to it.
T h e b e st means of controlling loose smut is through the use of certi-fied smut-free seed. Whe n s e ed is known to b e infected with loose smut mycelium, the best way of disinfecting it is by treating it with hot water. Usually small lots of s e ed are treated with hot water and planted in isolated fields to produce smut-free s e ed to b e u s ed during the next season. T h e hot-water treatment consists in soaking the seed, contained in half-filled burlap bags, in 2 0 °C water for 5 hours, drain-ing it for 1 minute, dippdrain-ing it in 4 9 °C water for about 1 minute, then in
Bunt, or Stinking Smut, of Wheat
Bunt, or Stinking Smut, of Wheat Occurrence and Importance
Bunt, or stinking smut, of wheat, sometimes called covered smut of wheat, is widely distributed in all the wheat-growing areas of the world. In the United States, the d i s e a se causes its greatest losses in the Pacific Northwest.
Bunt affects plants by destroying the contents of infected kernels and replacing them with the spores of the fungus. Bunt also causes slight to severe stunting of infected plants, d e p e n d i ng on the particu-lar species of the bunt fungus involved. Infected plants are usually more susceptible than healthy plants to certain other diseases and to winter injury. Bunt causes losses in grain yields proportional to the 52°C water for exactly 11 minutes, and immediately afterward in cold water for the s e ed to cool off. T h e s e ed is then allowed to dry so that it can b e sown. Since some of the s e ed may b e killed by the hot-water treatment, a higher s e e d i ng rate may b e e m p l o y ed to offset the re-d u c ere-d germinability of the treatere-d seere-d.
Selected References
Brefeld, O., a nd R. Falk. 1905. " D ie Bluteninfektin b e i d e n B r a n d p i l z en u n d d ie natur-liche Verbreitung der B r a n d k r a n k h e i t e n ," 74 p p. H e i n r i ch S c h o n i n g h, Munster i.W.
E d g i n g t o n, L. V . a nd E . R e i n b e r g s. 1966. Control of loose s m ut in b a r l ey with s y s t e m ic fungicides. Can. J. Plant Set. 46: 336.
F r e e m a n, Å. M., a nd E . C. J o h n s o n. 1909. T h e loose smuts of barley a nd wheat. U.S.
Dept. Agr. Bur. Plant Ind. Bull. 152: 4 8 p p ., illus.
K a v a n a g h, T. 1961 . T e m p e r a t u re in relation to loose smut in barley a nd wheat. Phyto-pathology 51: 189-193.
L e u k e l, R. W. 1936. Factors influencing infection of b a r l ey b y loose smut. Phytopathol-ogy 26: 6 3 0 - 6 4 2 .
L e u k e l, R. W., a nd V. F. T a p k e. 1955. B a r l ey d i s e a s es a nd their control. U.S. Dept. Agr.
Farmers' Bull. 2089: 2 8 p p.
P o p p, W. 1 9 5 1 . Infection in s e e ds a nd s e e d l i n gs of w h e at a nd b a r l ey in relation to d e-v e l o p m e nt of l o o se smut. Phytopathology 41: 2 6 1 - 2 7 5 .
P o p p, W. 1958. An i m p r o v ed m e t h od of d e t e c t i ng loosesmut m y c e l i um in w h o le e m -bryos of w h e at a nd barley. Phytopathology 48: 6 4 1 - 6 4 3 .
Von S c h m e l i n g, B., a nd M. Kulka. 1966. S y s t e m ic fungicidal activity of 1,4-oxanthiin derivatives. Science 152: 6 5 9 - 6 6 0 .
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number of plants affected and may vary from one location to another.
Whe n bunt is not controlled, it may cause devastating losses, but even with the effective control measures practiced in the United States to-day, the d i s e a se causes annual losses of more than 20 million dollars.
In addition to destroying the kernels of infected plants, bunt causes market losses by reducing the quality, and the price, of wheat contam-inated with smutted kernels or smut spores b e c a u se of the discolora-tion and the foul odor they impart to the whole wheat crop. Bunt, moreover, results in explosions in combines and elevators during threshing or handling of smutted wheat b e c a u se of the extreme com-bustibility of the oily smut spores in the presence of sparks from ma-chinery.
Symptoms
Plants infected with the common bunt fungi are usually a few to several centimeters shorter than healthy plants and may sometimes b e only half as tall. Plants infected with the dwarf bunt fungus may b e only one-fourth as tall as healthy plants. Infected plants may appear slightly bluish green to grayish green in color and may show s o me flecking or chlorotic mottling on the leaves, but this is not easily dis-tinguishable from the normal green color of healthy plants. T h e root system of infected plants is usually poorly d e v e l o p e d, and tillering may sometimes b e increased.
Distinct bunt symptoms, however, appear when the heads of in-fected plants e m e r g e from the boot. Their color is usually bluish green rather than the normal yellowish green, they are slimmer than healthy heads, and the glumes s e em to spread apart and form a greater angle with the main axis of the h e ad than they do in healthy plants (Figs. 50 and 51). Infected kernels are shorter and thicker than healthy ones and are grayish brown rather than the normal golden yellow or red.
Whe n these kernels are broken following their maturation, they are found to b e full of a sooty, black, powdery mass of fungus spores which give off a distinctive odor resembling that of decaying fish. Dur-ing harvest of infected fields, large clouds of spores may b e released in the air.
The Pathogens: Tilletia caries and Ã. foetida
T h e fungi causing bunt or stinking smut of wheat are basidiomyce-tes. Another closely related species, T. contraversa, also causes bunt.
Bunt, or Stinking Smut, of Wheat 307
F i g. 50. C o v e r ed smut (A) a nd loose smut (B) of barley.
(C) Healthy head. (Photo by courtesy of U . S. D e p t. Agr.)
All three species are similar in their life histories and d i s e a se develop-ment, but the teliospores of T.foetida have smooth walls, while those of T. caries have highly reticulate walls. T. contraversa is the cause of dwarf bunt, and its spore walls have large, polygonal reticulations.
T h e mycelium of the pathogen is hyaline and its cells contain two nuclei. During sporulation most mycelial cells are transformed into almost spherical, brownish, binucleate teliospores, which are thick-walled and m e a s u re 15-23 μ in diameter, while the rest of the myce-lial cells remain hyaline, thin-walled, and sterile. U p on germination of a teliospore a b a s i d i um is produced, at the e n d of which 8-16 basid-iospores develop. T h e basidbasid-iospores are long, hyaline, and uninu-cleate. T h e y are usually called primary sporidia. T h e primary sporidia
F i g. 5 1 . Bunt, or stinking smut, of wheat. Left two h e a ds show abnormal s p r e ad of the g l u m es d ue to the bunt balls. Right, healthy h e a d. (Photo by courtesy of U.S. D e p t.
Agr.)
fuse in pairs through production of lateral branches b e t w e en compati-ble mating types of primary sporidia and appear as Ç-shaped struc-tures (Fig. 52). T h e nucleus of each primary sporidium divides, and through exchange of one of the nuclei the two fused primary sporidia b e c o me dikaryotic. Whe n these primary sporidia germinate, they pro-d u ce short hyphae on which pro-dikaryotic seconpro-dary sporipro-dia are formepro-d.
Fig. 52. Disease cycle of bunt or stinking smut of wheat caused by Tilletia caries.