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Bacterial alfalfa leaf spot


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The bacterial leaf spot described here was first recognized on Turkestan and Grimm alfalfa (Medicago sativa L.) in two locations at Madison, Wis., in August 1930. The alfalfa in both instances was in cultivated experimental rows and had been repeatedly splashed bywatering with a garden hose. In October 1931 the disease recurred under similar circumstances at one of these locations, but with the abandonment of alfalfa growing at these places the disease has not been seen during the three following summers. At no time has it been found in alfalfa fields. These observations, together with the results of inoculations, suggest that while the disease appears potentially destructive, it does not often find favorable conditions for development in this locality. Inasmuch as it may develop elsewhere and may easily be mist


A bacterial leaf spot of alfalfa, apparently new to science, has been observed at Madison, Wis. The symptoms and isolation and inoculation studies are described. On the basis of the morphological and physiological characters of single-cell isolations from six cultures of the organism, itis described as a new species and the name Phytomonas alfalfae is suggested.


This leaf spot, like most bacterial leaf spots, is at first very small and water-soaked in appearance. As the spots increase in size, they may coalesce, especially along the midrib and at the ends of the leaflets, forming areas of dead tissue which soon dries. The dry center of spots attaining a diameter of 2 to 3 mm is often yellow with a darkbrown border surrounded by a straw-colored halo. Smaller lesions may appear merely as dark-brown spots. Characteristic leaf injury from this disease is shown in figure 1. No stem lesions have been observed except following inoculations. Thus, only in the early stages of its development does the disease suggest its bacterial origin. At that time it is easily distinguished from leaf infections of the bacterial stem bUght caused by Phytomonas medicaginis (Sack.) Bergey et al. by the absence of the stem lesions characteristic of that disease and also by the small size of the spots, which have not been observed to extend to form the large yellow areas described for that disease. After the early water-soaked condition has. passed, this leaf spot resembles certain fungous leaf spots. The smaller dark-brown spots are not easily distinguished from partly developed spots caused by Pseudopeziza medicaginis (Lib.) Sacc. The larger bordered spots may closely resemble those caused by Pseudo-plea hriosiana (Poll.) Hoehn. Because of the strong resemblance of this bacterial spot to lesions caused by these two fungi, it may have occurred unrecognized many times in the past.


Six separate isolations of the organism were made in 1930 with the usual poured-plate technic. The pathogenicity of the cultures obtained was proven on plants in the greenhouse, the bacteria were reisolated, and the pathogenicity of the cultures obtained_again_demonstrated by inoculation.


The plants inoculated were kept in a closed glass chamber for half a day before the leaves were sprayed with the bacterial suspension in distilled water, and were left in the chamber for a half day after this inoculation. In soine cases a trace of castile soap was added to the bacterial suspension in order to facilitate wetting the leaves. Under these conditions infection was often far more destructive to the plant than natural infection observed in the field. Puncture inoculations were also made, which, though successful, were not commonly as satisfactory as those made by spraying the leaves. The cultures were carried in stock until 1933, when their pathogenicity was again demonstrated by greenhouse inoculations. Following this, single-cell isolations were made from the six different cultures.^ The pathogenicity of these single-cell isolations was demonstrated soon after they were made and again after they had been carried in stock for several months.


The bacteriological characters of these six single-cell cultures were determined according to the common procedures. Unless otherwise noted, the methods employed were those given in current (1933) revisions of publications by the Committee of Bacteriological Technic of the Society of American Bacteriologists.^ Each of these tests was made in triplicate with suitable controls for each of the six cultures. Unless otherwise noted, each test was performed at least a second and usually a third time.