Frenching of Ragweed (Ambrosia artemisiifolia L.)

Abstract

Typical frenching of tobacco (Nicotiana tabacum L.), as described by Wolf (8), is recognized as producing short internodes, loss of apical dominance, and the development of a rosette of many leaves (polyphylly), giving a witches'-broom effect. The first symptoms appear as a pinhead chlorosis, followed by very chlorotic, newly developing leaves which narrowly elongate with wavy margins. It is possible that the tobacco plant (Nicotiana tabacum L.) is the best indicator plant to determine the presence of the frenching factor in the soil. This peculiar development of the leaves of tobacco has never been found to be associated with a pathogenic bacterium, a fungus, or a virus. It is generally recognized as a nonparasitic disease and therefore considered physiological. Steinberg (5, ?) has worked with many tobacco soil organisms which are not considered pathogenic but which might produce a diffusate sufficiently toxic to produce the symptoms of frenching. One of these, Bacillus cereus Frankland and Frankland, was found to be capable of producing the frenching symptoms. Steinberg (7) has found this organism to be present in greater numbers in the rhizosphere of tobacco in soils that contained frenched tobacco than in the soils with normal plants. The origin of this disease is some soil factor. The frenching factor is not in all soils but has been found, in particular, in soils which are known to remain moist during a prolonged drought period, even though tobacco has never been grown in such soils. Continuing the work previously reported (2, 3), it has been demonstrated that the frenching factor, if present, can be made to show its presence by growing tobacco plants at a soil temperature of 35? C. Usually within a period of 5 to 15 days frenching symptoms appear in the newly developing leaves, becoming more distinct and severe with succeeding leaves. When the soil temperature is 21? C, or less, the presence of the frenching factor does not become evident.