Ecology and chemical control of purple stain caused by Cercospora kikuchii on soybean seed

Since soybean (Glycine max Merr.) cultivation in the converted paddy fieldes was recommended, its cultivation area remarkably increased from the late of 1970s. Along with the increase of cultivation area, diseases occurrence became the serious problem for soybean production. The occurrence of mosaic, bacterial pustule (Xanthomonas campestris pv. glycinea (Nakano) Dye), purple stain of seed (Cercospora kikuchii Matsumoto et Tomoyasu) and downy mildew (Peronospora manshurica (Naumov) Sydow ex Gaumann) was observed everywhere in Hiroshima Prefecture. Among these diseases, purple stain of seed caused the severest damage to soybean in Hiroshima Prefecture. Although the disease affects every organ on soybean, the most serious damage is to discolored seed coat purple and objectionable in market. The disease symptoms, except for the characteristic ones on seeds, were not easily distinguished by naked eye from those of other spot disease. To clarify the progress of purple stain of soybean from the germination stage to harvest stage, conidial formation of causal pathogen in the sampled lesions was observed under the microscope in this studies. these studies were carried out to develop the effective control methods for the disease on seeds, purple stain of soybean, based on cultural and chemical control measures. 1. Ecology of purple seed stain of soybean 1) Disease incidence on soybean seeds in farmers' fields under different culture environments The disease incidence on seeds in early sowing fields (sown before July) was severer than that in late sowing ones (sown after July). The diseased seeds were more in fields situated near mountainous area than those in plain fields. The occurrence of disease on leaves in early July significantly promoted the spread of seed infection. The diseased seeds, however, was remarkably depressed in chemical sprayed fields. There was little difference of the disease incidence on seeds among harvesting dates and between converted paddy fields and upland fields. No significant differences in the level of disease incidence on seeds was found among cultivating regions and varieties. 2) Progress of disease on soybean plants during cultivation (1) When the diseased seeds were sown in field, the disease initially occurred on cotyledons and then gradually disseminated to the newly-developed soybean organs with the lapse of time. The constant exposure of the newly-developed organs to dispersed conidia from already infected organs allowed infection to continue. (2) Although the manner of disease dissemination was the same in both early and late-sowings, the degree of infection on each organ was severer in the early-sowing. The results of our field experiment study as well as those of the farmers' fields' survey strongly indicate that the late-sowing of soybean could be a promising cultural practice for the control of purple stain of soybean. (3) When the diseased seeds were sown in field, the incidence of infected seeds was first observed at about 40 days after flowering (seed development 40 to 60%). The number gradually increased thereafter to reach the maximum at 70 to 75 days after flowering (10 to 15 das before the harvest stage, seed development 100%). (4) The diseased seeds, i. e., the purple discolored seeds were first observed at about 70 days after flowering (about 20 days before the harvest stage, seed development 100%). They rapidly increased within 10 days. From these facts, it is apparent that the symptom on infected seeds, purple discoloration on seeds, does not appear with the passage of latent period of disease but rather appear within few days after the time when seeds develop into almost full sized green beans and their water content decreases physiologically, irrespective of the infection time. (5) The incidence of infected and diseased seeds was influenced by seed development stages. It seems, therefore, that the progress of infection and disease on seeds in relation to their development stages, as well as to the number of days after flowering is necessary. 3) The pathogen overwintered as stromata on the lesions of diseased plant debris left on field. Plenty of conidia were produced on some of these debris even in the next spring. However, the diseased plant debris were plowed under just after the harvest of previous soybean, most of them decomposed until the next spring and the pathogen on them survived extremely little. 2. Control of seed disease caused by purple stain of soybean 1) Cultural control of purple stain of soybean (1) The disease spread from a diseased plant (a single infection source) to the healthy plants In the cultivation that the disease free seeds contaminated with a diseased seed (an infection source) were sown in field, the disease dissemination delayed and the disease incidence at the early epidemic stages was slight compared to that in sowing the diseased seeds. However, the disease spread from an infection source to healthy plants, the diseased plants gradually increased around an infection source. At the beginning of pod and seed [evelopment stages, many plants were infected and had many diseased leaves. Because these leaves became the infection sources for pods and seeds, they were infected severely at the harvest stage like the case sown disaesed seeds. It is apparent from this result that sowing the disease free seeds is an important cultural control measure for purple stain of soybean. (2) Infection from diseased defoliated leaves, infection sources, during cultivation In soybean cultivation where diseased defoliated leaves were removed from field, the disease incidence on growing leaves, pods and seeds was remarkably suppressed comparing to ordinary cultivation where diseased defoliated leaves were left in the field. Since the removal of defoliated leaves from field is not practical, covering them with soil during weeding and intertillage is considered to be a viable cultural control measure for purple stain of soybean. This practice would prevent the dispersion of conidia from diseased defoliated leaves, thus, reducing the inoculum potential in field. (3) Pathogenicity of the pathogen overwintering on diseased plant debris As already stated, the pathogen overwintered as stromata on the lesions of diseased plant debris left on field. The pathogen overwintered on these debris kept pathogenicity until the next soybean cultivation season unless these debris decomposed. However, diseased plant debris were plowed under just after the harvest of previous soybean, |ost of them decomposed until the next spring and the pathogen on them lost pathogenicity. It is apparent from this result that plowing diseased plant debris under just after the harvest of previous soybean is an important cultural control measure for purple stain of soybean. 2) Chemical control of purple stain on soybean seed (1) Thiophanate-methyl protected seeds from infection by the pathogen of purple stain of soybean. It also had a curative effect by killing the internally borne pathogen in the infected seeds or by suppressing its growth below the level necessary to develop the purple discolorations and controlled the disease on seeds through harvest stage. The results of infection time and rate of seeds, residual effect and mode of action of thiophanate-methyl indicated that a one-time application between 15 and 50 days after flowering significantly suppressed disease incidence on seeds through harvest stage. As already stated, the incidence of infected and diseased seeds was closely relates to seed development stages. Therefore, it is necessary to express the optimum timing of chemical application according to the seed development stages along with the days after flowering. (2) More than six applications of thiophanate-methyl during the soybean growing period stimulated the development of resistant strains. The disease on leaves and seeds infected with the resistant strains was not suppressed by thiophanate-methyl. Therefore, the application of thiophanate-methyl needs to be restricted to one properly timed application in a cultivation season to avoid the development of resistant strains. (3) Isolation frequencies of thiophanate-methyl-resistant strains of Cercospora kikuchii in farmers' fields of each location ranged from 0 to 10% with an average of 4.4% of the total sampled isolates. However, the resistant strains were distributed over a fairly wide area in Hiroshima Prefecture. Cultural control measures, such as sowing disease free seeds, plowing under diseased plant debris immediately after harvest and covering the diseased defoliated leaves with soil during weeding and intertillage, are effective to suppress the disease incidence on seeds, purple stain of soybean. Late-sowing of soybean is also a promising cultural practice for the control of purple stain of soybean. The effect of thiophanate-methyl seems to be enhanced in combination with these cultural control measures.