Herbs and Other Special Crops

C. A. Thomas.

Several hundred varieties and species of plants are put to specialized uses by man. Many are cultivated in a limited way and on a now-and-then basis according to demand.

This miscellaneous botanical assortment often is referred to collectively as special crops because the plants are not known to many people. Usually only a few growers are favorably situated to cultivate one or more of them, and usually a single location is suited only to a few of them. They are not unimportant, however, and their areas of production may be expanded when their end uses expand and when varieties adapted to other localities are available.

Among the special crops are herbs and condiments, as peppermint, spearmint, sage, caraway, thyme, marjoram, and coriander, which yield flavoring compounds. Plants yielding compounds of medicinal use are another important group. Among them are digitalis, belladonna, poppy, and ergot, which contain alkaloids or glucosides. Sumac and canaigre are sources of tannin. Several species of plants yield compounds that have insecticidal value. Seeds of castor bean, safflower, sesame, and perilla contain oils of use for food or industry.

The diseases affecting the special crops are similar to and as diverse as those affecting other crops. Root rots, wilts, leaf spots, blights, and other diseases are common among them. Numerous viruses and species of fungi and bacteria are among the agents causing diseases.

A knowledge of the general cultural methods, values, and size and location of the areas of production of these crops is necessary for a clear understanding of their diseases and methods of control.

The production of many of the special crops requires considerable labor, special methods of handling, and often special equipment. Several of the crops must be planted, weeded, harvested, and processed by hand. The amount of labor involved is usually the limiting factor of production for any one grower. Special drying, grinding, or distilling equipment may be required by the grower to prepare his product for market. We thus find many of the special crops being produced year after year by the same few growers who possess the necessary skill, labor, and equipment. Certain perennial crops, like peppermint and spearmint, are usually grown for several years on the same land to avoid replanting costs. Crop rotation then is at a minimum. Persons interested in special crops as a side line or hobby usually follow no rotation practice.

Contrary to popular opinion, the culture of many special crops is not a highly profitable enterprise on a longtime basis. Except in times when imports are cut off or seriously reduced, many of our botanical drugs, condiments, and oils are imported from Europe, Asia, or the Tropics at extremely low costs.

Collections of wild plants are adequate to meet the demand for several plants of specialized use and a few hundred acres suffice for most of the cultivated species. The industrial uses of products of peppermint and spearmint, safflower, and castor bean, however, require the production from several thousand acres.

A considerable amount of seed and planting stock of many special crops are brought into the United States from time to time from foreign countries.

Such imports result from efforts to establish production of crops not native to the United States and to find sources of germ plasm for improvement of crops now in production. The imports often result in the introduction of new disease organisms, which are a source of trouble for the grower.

The use of fungicides, particularly for controlling foliage diseases where several applications are involved, or expensive soil fumigants for controlling soil-borne organisms is not economically feasible for the low-value crops. Fungicidal residues cannot be tolerated in the case of certain medicinal plants, such as digitalis, where the foliage may be used directly as a medicinal compound.

It is often possible to grow the crops in sections where diseases are least troublesome since production in most cases is on a limited scale.

Certain pathogens may be excluded or eradicated by special seed, plant, or soil treatments. Such measures are sometimes effective with the special crops because of the isolated nature of the plantings. Proper cultural practices and treatment of seed are usually helpful.

The use of resistant varieties, if they are available, is often the best or only method of control. Their development, however, is difficult, time consuming, and sometimes impossible.

OF THE MINT DISEASES, let us consider first the verticillium wilt of peppermint. Peppermint, Mentha piperita, is cultivated in the United States primarily for its essential oil, which is used to flavor chewing gum, dentifrices, and candy. It also is used in pharmaceutical preparations. The crop is grown in this country in two areas, Indiana and Michigan, and Oregon and Washington. The highest level of production was obtained in 1947, when 1,502,000 pounds of oil of an approximate farm value of 10 million dollars were produced on 47,018 acres.

Peppermint is a shallow-rooted, soil-depleting, long-day plant, which rarely produces viable seed because of its sterility. Propagation is accomplished in the spring by transplanting runners and rhizomes pulled or dug from the soil and by transplanting shoots, which are pulled up with some adhering roots. Peppermint is cultivated as a row crop the first year after planting. The row mint is plowed 4 to 6 inches deep in the late fall of the first year. In subsequent years the mint is allowed to spread over the field and produce a solid stand, called meadow mint. The same planting of mint may be maintained for several years, but it is usually unprofitable to keep a field longer than 2 or 3 years.

Wilt, caused by Verticillium alboatrum var. menthae, is probably the most serious disease of peppermint in the United States. It was identified in Michigan in 1924, but undoubtedly it was present before then. It has been most destructive in Michigan and Indiana, but it has been reported also in Oregon and Washington. The disease in Michigan has been one of the major factors responsible for the reduction of peppermint acreage from more than 21,000 in 1941 to less than 10,000 in 1951.

The wilt causes a direct loss by reducing the yield of oil. Infected plants may be killed, stunted, or defoliated. They are weakened and less able to overwinter. Thin stands of mint often are the result.

All species of the genus Mentha tested have been found to be susceptible to the wilt disease. Peppermint is one of the most susceptible species. Several varieties of spearmint are more resistant. The organism causing the mint wilt disease is rather specific for mint. Only one or two plants that are not species or varieties of Mentha are susceptible.

The most characteristic symptoms Of wilt in peppermint and spearmint are asymmetrical growth of the leaves dwarfing, chloranemia, or a bronze to Purple coloration of the leaves, wilting, and the development of cankers on the stems and rhizomes.

The causal organism is primarily a vascular-invading fungus pathogen that may survive in soil for several years even in the absence of mint. The fungus may invade young roots of the plants directly through epidermal cells or through wounds. Once inside, the mycelium may become systemic in the vascular system. The exact method by which the organism causes wilting and the development of the other symptoms is not completely understood. Mechanical plugging of the water-conducting elements is not considered wholly responsible. Toxins probably are involved.

High soil moisture at a soil temperature of approximately 79 F. is considered likeliest for maximum infection. Development of the disease following infection is favored by high soil temperature and low soil moisture. Wilt is most severe during hot, dry seasons. Ample soil moisture for infection is presumably present in the early part of most seasons.

Several factors were responsible for the development of the wilt disease in the Midwest to such proportions that culture of peppermint was abandoned in many areas considered to be most suitable for peppermint growing and the production of high-quality oil.

The intensive culture of the plant in a limited area along with the practice of growing mint 2 or 3 years or more on the same land without rotation were undoubtedly conducive to a build-up of the organism in the soil. The fungus was easily distributed over the area on planting stock from infested fields and no real effort was made to exclude the organism from new fields by obtaining planting stock from disease-free plants.

Wilt has not become a serious disease of peppermint in Oregon and Washington. The presence of the disease there is easily explained, because the industry there was started with plants obtained from the Midwest. The fact that wilt has not developed to any extent in the Pacific Northwest, where mint culture is more intensive than in the Midwest, may indicate that environmental conditions are of primary importance in development of the disease.

Eradication of the verticillium wilt organism in the soil by fungicides or soil fumigants is not a promising method of control. Maintenance of soil moisture at proper levels, crop rotation, and avoidance of root injury are helpful in reducing injury from wilt. The use of a wilt-resistant type of peppermint appears to be the most satisfactory method of control.

It is possible to select wilt-resistant types from hybrids produced by crossing M. piperita with the wilt-resistant M. crispa. Some selections made at Michigan State College are reported to yield considerably more oil than M. piperita, to be very resistant under average conditions, and to be similar in quality to natural peppermint.

The manufacturers of chewing gum and candy demand peppermint oil of the highest quality because the sale of their products depends upon flavor. They are reluctant to accept oil from any source other than M. piperita for fear of changing the flavor of their products. Until such time as the manufacturers are convinced that the oil from any disease-resistant type of mint exactly duplicates that of M. piperita, or until the supply of peppermint oil is exceedingly critical because of diseases, it is unlikely that any type of mint other than the one now being used will be accepted as a commercial source of oil.

RUST, caused by Puccinia menthae, has been recognized as a disease of spearmint for many years. The organism produces yellow or brown pustules on the stems, leaves, and petioles in spring or early summer. Later in the season the pustules turn darker brown. The disease reduces the yield of oil because affected leaves produce less oil and often are killed before harvest.

Dusting with sulfur or spraying with bordeaux mixture and early cutting of heavily infested fields have been recommended as control practices. Certain fungicides, such as the dithiocarbamates, are not suitable for controlling mint rust since the residue on the leaves decomposes when the mint is steam-distilled to obtain the oil. Decomposition of the residue results in the production of carbon disulfide, which is highly toxic and may impair the flavor of the oil.