Abstract:
A fungicidal compound having a general structure corresponding to a member selected from the group of 2,3-dicyano-5,6-dihydro-p-dithiin and its 5-alkyl derivatives, as expressed by the following general formula: ##STR1## wherein R stands for H or an alkyl radical of C 1  - C 6 .

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to novel compounds adapted for use in the control of plant diseases. 
     The invention further relates to a fungicidal composition for use in the same purpose as above, and including as its effective substance at least one of said compounds, as will be later more fully described. 
     2. Object of the Invention 
     The main purpose of the present invention is to provide the above kind of compounds and compositions which rapidly decompose upon being applied onto the plant, thus representing no appreciable residual characteristic remaining per se in the earth and thus no apparent adverse and poisonous effects upon human bodies. 
     SUMMARY OF THE INVENTION 
     This invention resides in fungicidal compounds and compositions adapted for the control of plant diseases, said compositions having, as their effective substance, at least a member selected from the group consisting of 2,3-dicyano-5,6-dihydro-p-dithiin as expressed by the following general formula: ##STR2## wherein R stands for H or an alkyl of C 1  -  C 6 , and its 5-alkyl derivatives. 
     The aforementioned compesitions are highly effective for the control of various plant diseases, for example, powdery and downy mildews of leaf- and fruit vegetables; and rice blast. They are specifically and superiorly effective for the control of plant mildew diseases, recently raising the problem of strong and irreparable immunoreactions to substantial known fungicides. 
     As will be later more fully described, the aforementioned chemical compounds are highly safe to living plants. As an example, they do remarkably not represent any harmful effect, even in the highest possible atmospheric temperature in summer seasons, as high as 40° C, upon living plants, thus being astonishingly superior as the active agent for the above purposes, especially for the fungicidal control of house-cultured plants. 
     According to our experimental results, the aforementioned compounds have a strong tendency of early decomposition upon once applied to affected living plants, thus giving no rise to extended residence on plants and in soil. Therefore, the practical use of these compounds does not provide any fear of harmful effect upon manipulating personels by a residual amount of the fungicide, or of polution of the related environment. 
     The above compounds are novel, with exception of 2,3-dicyano-5,6-dihydro-p-dithiin and 2,3-dicyano-5,6-dihydro-5-methyl-dithiin. 
     In order to prepare the above compounds, dialkali salt of 1,2-dicyano-1,2-dimercaptethene is reacted with 1,2-dihalogenoalkane, without difficulty, and indeed in accordance with the following formula: ##STR3## wherein M stands for an alkali metal; 
     X stands for a halogene atom. 
     The preparation can be carried out, as a representative example, in the following way: 
     DETAILED DESCRIPTION OF THE INVENTION 
     Preparation of 5-propyl-2,3-dicyano-5,6-dihydro-p-dithiin (Compound IV) 
     9.3 g (0.05 mole) of di-sodium salt of 1,2-dicyano-1,2 -dimercaptethene and 40 ml of dimethyl formamide (D.M.F.) were introduced in a reaction flask, of 100 ml, fitted with agitator, and then, 20 ml of D.M.F.-solution including 11.5 g (0.05 mole) of 1,2-dibromopentane were added dropwise at room temperature under agitation to the reaction mixture for 30 minutes. 
     Upon finishing the dropwise addition of said D.M.F.-solution, the reaction mixture was agitated for a further 2 hours, and the liquid state reaction mixture was treated in a reducing evaporator, so as to reduce its volume to about one fifth and the condensed liquid mixture was added to 200 ml of water, thereby precipitating yellow brown solids. These solids were filtered off, washed with water, and recrystallized from methanol to 7.3 g of white yellow crystals, m.p. :70° - 71° C. Yield: 69.5%. 
     In the similar way, other compounds I-III, V-VIII can be easily prepared. 
     These representative compounds I - VIII are listed in the following Table 1. 
     The infra red absorption curves of these compounds I - VIII are shown on the accompanying drawings. 
     
                       Table 1______________________________________Compound                             m.p.No.      Formula           Shape     (° C)______________________________________     ##STR4##         Colorless crystals                                134 - 5II     ##STR5##         colorless crystals                                86 - 7III     ##STR6##         colorless crystals                                34 - 5IV     ##STR7##         white yellow crystals                                70 - 1V     ##STR8##         colorless needles                                86 - 7VI     ##STR9##         white yellow oil                                --VII     ##STR10##        white yellow oil                                --VIII     ##STR11##        colorless crystals                                44 - 6______________________________________ 
    
     These compounds may be used per se, or preferably in the form of powdery composition, emulsion, suspension or solution, as the case may be, by addition of conventionally known carrier or diluent. 
     In the practical use of the fungicidal composition according to this invention, if necessary, a known extender, emulsifier, surfactant, wetting agent and/or the like auxiliaries may be added. Other kinds of fungicides, proper bacteriocides, insecticides and/or fertilizers may be combined in use, without deterioration of the fungicide according to this invention and with no fear of decomposition of such additives. 
     In the following, several representative fungicidal compositions according to this invention will be raised. However, it should be noted that the invention should not be limited only to these specifically selected embodiments. Kinds and ratios of the carriers, diluents, additives and auxiliaries may be varied in a broad range. Given parts are by weight, if not otherwise specified. 
    
    
     EXAMPLE 1 -- POWDERY COMPOSITION 
     
         ______________________________________compound No. II    5 parts;clay              40 parts;talc              57 parts______________________________________ 
    
     These materials are mixed together evenly and pulverized into a powdery composition ready for use. 
     EXAMPLE 2 -- AQUEOUS COMPOSITION 
     
         ______________________________________                parts______________________________________compound No. VI        50polyoxyethylene alkylaryl ether                   6kieselguhr             44______________________________________ 
    
     These materials are mixed well with each other and added with water to provide a ready-for-use aqueous composition. 
     In the following, several experimental results will be given for the demonstration of superior fungicidal effects of the above mentioned compounds: 
     Experiment 1: Control of Powdery Mildew of Cucumber (cucumis sativus) 
     A number of flowerpots, each being of 10 cm-diameter, were used to culture cucumber plants, &#34;Sagami-Hanjiro&#34;, a Japanese cucumber variety, one cucumber per one pot. Then, the plants were treated with aqueous suspensions of the hydrated composition mentioned in the foregoing Example 2 at different concentrations as per the numerical data to be given in the following Table 2. As the reference agent, called &#34;control A&#34;, 6-methyl quinoxaline-2, 3-dithio carbonate was used in the similar way. All the cucumber leaves thus conditioned and dried, were inoculated in a green house with spores of Sphaerotheca fuliginea Pollacci by means of a hair pencil. After seven days following the inoculation, the affected leaves were visually inspected and classified in accordance with the following grades. 
     
         ______________________________________grade   degree of affection______________________________________0       unaffected0.5     less than 10% area of inoculated   leaves affected1       10-20% area of inoculated leaves   affected2       20-40% area of inoculated leaves   affected3       40-60% area of inoculated leaves   affected4       60-80% area of inoculated leaves   affected5       more than 80% area of inoculated leaves   affected______________________________________ 
    
     
                       Table 2______________________________________                            harmful                            effect of      Concentration                 mean degree                            application ofCompound No.      ppm        of affection                            the chemical______________________________________I          500       0          NoneII         500       0          &#34;III        500        0.5       &#34;IV         500       0          &#34;V          500       0          &#34;VI         500       0          &#34;VII        500       0          &#34;control A   62.5     0          &#34;non-conditioned      --        5          --______________________________________ 
    
     Experiment 2: Control of Downy Mildew of Cucumber (cucumis sativus) 
     A number of flowerpots, each being of 10 cm-diameter, were used as before to culture cucumber plants, &#34;Sagami-Hanjiro&#34;, a Japanese cucumber variety, one cucumber per one pot. Then, the plants were treated with aqueous suspensions of the hydrated composition mentioned in the foregoing Example 2 at different concentrations as per the numerical data to be given in the following Table 3. As the reference agent, called &#34;control B&#34;, tetrachloroisophthalonitrile was used in the similar way. All the cucumber leaves thus conditioned and dried, were inoculated in a green house with spores of Pseudoperonospora cubensis Rostowzew by means of a hair pencil and kept at 22°-23° C in a highly humid atmosphere for 24 hours. After five days following the inoculation, the affected leaves were visually inspected and classified in the same way as adopted in the foregoing Experiment 1. The results are given in the following Table 3. 
     
                       Table 3______________________________________                            harmful                            effect of      Concentration                 mean degree                            application ofCompound No.      ppm        of affection                            the chemical______________________________________I          500       0          NoneII         &#34;         0          &#34;III        &#34;          0.5       &#34;IV         &#34;         0          &#34;V          &#34;          0.5       &#34;VI         &#34;          0.5       &#34;VII        &#34;          0.5       &#34;VIII       &#34;         0          &#34;control B  750       0          &#34;non-conditioned      --        5          --______________________________________ 
    
     Experiment 3: Control of rice blast. 
     A number of flowerpots were used to cultivate water rice plants, &#34;Sasanishiki&#34;, a Japanese variety, in one-to-one relationship, and these plants were conditioned with an aqueous suspension of the hydrated compound set forth in the foregoing Example 2 at various concentrations given in the following Table 4, to such a degree that all the leaves of the plants were well wetted. Then, after being dried, the rice leaves were inoculated with an aqueous suspension of spores of Pyricularia oryzae Cavara by means of a spray gun and kept at 27°-28° C in a highly humid atmosphere in a green house. 
     Upon the lapse of four days after said inoculation, all the leaves were inspected visually for finding out the affected spots on the leaves. The control power, %, was calculated from the following formula: ##EQU1## 
     The results are given in the following Table 4. As the reference compound, O, O-diisopropyl-S-benzyl thiophosphate, which is called &#34;control C&#34; in the same Table. 
     
                       Table 4______________________________________Com-   Concen-  number of control                            harmful effectpound  tration, affected  power, of applicationNo.    ppm      spots     %      of the chemical______________________________________I      500      21        97.5   None           (58)      (92.4) (&#34;)II     &#34;        12        98.6   &#34;           (112)     (85.4) (&#34;)III    &#34;        129       84.6   &#34;IV     &#34;        11        98.7   &#34;V      &#34;        23        97.2   &#34;VI     &#34;        9         98.9   &#34;VII    &#34;        8         99.0   &#34;VIII   &#34;        15        98.2   &#34;Control C     480      24        97.1   &#34;  (480)    (130)     (83.0) (&#34;)non-con-ditioned  --        83.6            --  (--)      (76.5)          (--)______________________________________ Remarks: parenthesized numerals, were obtained with another similar experiment carried out at a different season. 
    
     Experiment 4: control of Helminthosporium Rice Leaf Spots 
     Rice plants were cultured and controlled in the similar way as adopted in the foregoing Experiment 3. The inoculation was carried out with spores of Cochliobolus miyabeanus Drechsler. As the reference, 2,4-dichloro-6-(o-chloroanilino) 1,3,5-triazine, called &#34;control D&#34; was adopted. The control power was calculated by the foregoing formula adopted in Experiment 3. The inoculation was carried out by spraying and the inoculated plants were held at 27° - 28° C in a highly humid atmosphere. The visual inspection of the affected spots was made after four days following inoculation. 
     The results are given in the following Table 5. 
     
                       Table 5______________________________________Com-   Concen-  number of control                            harmful effectpound  tration, affected  power, of applicationNo.    ppm      spots     %      of the chemical______________________________________I      500      158       87.5   NoneII     500      132       89.6   &#34;Control D     625      171       86.5   &#34;non-con-ditioned  --       1,264     --     --______________________________________