By JAMES JOHNSON, Agent, Office of Tobacco and Plant Nutrition, Bureau of Plant Industry, and Professor of Horticulture, College of Agriculture, University of Wisconsin; and C. M. SLAGG, formerly Assistant Pathologist, and H. F. MURWIN, Agent, Office of Tobacco and Plant Nutrition, Bureau of Plant. Industry

During the last 10 years a disease of tobacco which is characterized by brown and decayed roots has been under observation and the subject of considerable experimental work. This condition has been referred to as brown root rot, to distinguish it from the more generally known black root rot due to Thielavia basicola (B. and Br.) Zopf. Intensive studies along etiological lines have failed to yield conclusive evidence as to the actual cause of the disease. Although most of the evidence points to a parasitic origin, other evidence apparently is contradictory to a parasitic hypothesis. The observations over a period of several years, together with the results from rotation experiments reported in this bulletin, show that the occurrence of brown root rot is closely correlated with the preceding crops grown on the land. Tobacco grown on sod is commonly most markedly affected, and the use of winter cover crops may bring on a similar condition. From an agronomic standpoint as well the problem becomes very complex, since the kind of preceding crop, the soil type, the environmental conditions, and other circumstances determine the occurrence and the severity of the disease.

So far as is known, brown root rot is most prevalent in the tobacco soils of the Connecticut Valley. A similar condition is known to exist in Wisconsin, Maryland, Kentucky, and in several other tobaccogrowing districts, but in the absence of a known parasite or other causal agency it is not readily demonstrable that the conditions observed are all due to the same cause.

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Other plants are affected by brown root rot, notably the tomato. Potatoes are affected to a lesser extent, as are many legumes. Little is known relative to the importance of this disease in the culture of other crops. In the case of tobacco culture in New England, however, it is to be ranked as one of the major maladies affecting the

crop.

The experimental work reported in this bulletin has been mainly conducted in the laboratories at the Wisconsin Agricultural Experiment Station, using soils from the Connecticut Valley, but the field plats were located in the Connecticut Valley. The results therefore apply more specifically to the soils of that valley, although in practice the conclusions are believed to have a wide application in tobacco culture.

FIG. 1.-A field of Havana tobacco grown at Suffield, Conn., showing a crop which is practically a failure, due largely to brown root rot. Note that some spots in the field are less seriously affected

Brown root rot occurs both in the shade-grown and sun-grown tobacco in the Connecticut Valley. Three varieties are here involved the shade-grown Cuban, the Havana Seed (Connecticut Havana), and the Connecticut Broadleaf, the latter two varieties being grown in the open field. The aboveground symptoms may vary somewhat with the different varieties and conditions. The most striking aboveground symptoms are the general stunting (fig. 1) and the temporary wilting of the affected plants during periods of high transpiration. The most striking cases of wilting have been observed in the Havana variety grown in the open field (fig. 2), although this may be equally common in the Broadleaf variety in New England. The shade-grown tobacco is not ordinarily so subject to wilting, apparently on account of the reduced transpiration under such cultural conditions. Under some conditions a gradual yellowing and death of the lower leaves of the plant occur, particularly during the late summer. The stunting, wilting, and yellowing are also characteristic of the Thielavia root rot and may, of course, be

caused by any other agency or condition interfering with the food or water supply of the plant.

The symptoms on the root system are more characteristic. Where the disease is extensive the plant is easily pulled from the soil, and the few remaining roots on the base of the stalk are brown and decayed; or, as is frequently characteristic in moderately stunted plants later in the season, a considerable number of well-developed roots with distinct lesions may be present in the uppermost layer of soil, but the taproot and strong secondary roots normally present in the deeper layers of soil are either lacking or have made little or no growth. These conditions might be mistaken for Thielavia root rot except for the absence of black lesions and spores of Thielavia. (Fig. 3.) Their absence in the presence of a root decay is one of the principal diagnostic characters for brown root rot. In both

FIG. 2.-A diseased field of Havana tobacco at Deerfield, Mass., showing characteristic wilting as a result of a deficient root system. The symptoms on the roots were typical of brown root rot

diseases the plants are apparently stimulated to send out more new roots than would otherwise develop from the portion of the stalk below the surface of the soil. These may or may not make considerable headway into the soil before being destroyed. The roots which are able to persist are on the average generally nearer the surface than the main fibrous roots of a normal plant.

So far as known the plants never actually die as a result of brown root rot but may remain throughout the entire season without making much growth. On the other hand, they may show a decided tendency to recover late in the season.

Brown root rot is not ordinarily distributed uniformly over large field areas. So far as observation has gone, the disease in its most serious form is often more or less limited to spots in the field; at other times, the whole field may be very uniformly affected except for occasional small spots of good tobacco. The lack of uniformity

of the plants is characteristic in fields after sod land in Wisconsin. Often the back furrow in a field is the only place where the tobacco grows normally. (Fig. 4.)

HISTORICAL REVIEW

So far as could be ascertained, the disease now referred to here as brown root rot of tobacco was undescribed at the time these studies were undertaken in 1915. One of the writers reported the occurrence of the trouble in 1919' (11). At that time it was believed that Fusarium might be the causal organism, since apparent mild infection was sometimes obtained with certain strains isolated from diseased hosts, although no heavy infection comparable to that occurring in the field was ever found. Clinton (4) in 1920 described a red root rot of tobacco in the Connecticut Valley which is undoubtedly the same as our brown root rot. Although red roots occasionally occur in this disease, it is believed that "brown root rot " is a better descrip

FIG. 3.-Lesions of brown root rot on roots of tobacco (A) and tomato (B). The dark-colored lesions shown here are actually brown rather than black. The localized lesions seem to bear a close relation to the point of attachment of secondary roots

tive name than "red root rot." Chapman (3) in 1920 also discussed the root-rot situation in the Connecticut Valley and recognized new disease distinct from black root rot. Neither of these writers, however, showed that any particular organism was definitely associated with the disease.

In 1924 the senior writer briefly described the disease in a general treatise on tobacco diseases (12).

Whether or not this disease has been described on crops other than tobacco can not be definitely ascertained. Occasional references in literature are to be found to obscure root diseases of various plants, attributed to Fusaria, Rhizoctonia, or other organisms, where the causal agency has not been definitely established.

The western tomato blight, for instance, ascribed by Humphrey (9) to Fusarium and by Heald (7) to Rhizoctonia, has some simi

The serial numbers in italic in parentheses refer to "Literature cited," at the end of this bulletin.