Synergistic fungicidal mixtures for the control of plant diseases
FIELD AND BACKGROUND OF THE INVENTION 
     The present invention relates to fungicides, more particularly to oomycetes controlling fungicides. 
     Although a wide variety of oomycetes controlling fungicides are known, the need exists for still more effective fungicides. 
     SUMMARY OF THE INVENTION 
     It has been found that the use of cymoxanil, trademark &#34;Curzate&#34; (U.S. Pat. No. 3,957,847), in association with dimethomorph is particularly effective in combatting or preventing fungal diseases. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention accordingly provides a method to combat fungal diseases in plants which comprises applying to the plants, seeds or soil, in admixture, a fungicidally effective aggregate amount of cymoxanil and dimethomorph. 
     Cymoxanil is a known fungicide. It has a local systemic fungicidal activity against, i.a., Plasmopara spp and Phytophthora spp. 
     Dimethomorph, which is disclosed in European Patent Application 120321, shows fungicidal activity with relatively low systemic activity against oomycetes such as Plasmopara spp and Phytophtora spp. 
     The use of dimethomorph in combination with cymoxanil surprisingly and substantially enhances the fungicidal effectiveness of cymoxanil, and vice versa. The method of the invention may particularly be useful to delay the development of dimethomorph-resistant oomycetes strains, especially field strains of Plasmopara spp and of Phytophtora spp. The method of the invention decreases the risk of fungal subpopulation developing resistance against dimethomorph. It is surprisingly effective against oomycetes strains which are sensitive or which have already developed resistance against phenylamide fungicides such as metalaxyl and oxadixyl. 
     The method of the invention is particularly suitable for use against fungi of the class oomycetes such as Phytophthora spp, Plasmopara spp, Peronospora spp, Pseudoperonospora spp, Sclerophthora spp and Bermia spp in crops such as grapevines, tomato, hops, cacao, tobacco, potato, peas, onions, cucurbits, crucifers or lettuce cultures or turf. 
     Examples of fungus/crop systems against which the method of the invention is particularly indicated are Pseudoperonospora cubensis in cucurbits, Plasmopara viticola in grapevines and most particularly Phytophthora infestans in potatoes and tomatoes. 
     The cymoxanil and dimethomorph may for example be applied in spray form, e.g., employing appropriate dilutions of a soluble concentrate or of a wettable powder formulation in water. 
     Suitable fungicidally effective aggregate amounts of cymoxanil and dimethomorph lie in the range of from about 550 to about 1000 g/ha of crop locus. In general, satisfactory results will be obtained when employing from about 300 to about 500 g/ha, e.g., about 400 g/ha of cymoxanil and from about 150 to about 400 g/ha, e.g., 250 g/ha, of dimethomorph. 
     The application rate may also be expressed in terms of concentrations. Spray liquors suitable for use in, for example, grapevines or potatoes comprise from about 50 to about 100 g per hectoliter. The spray treatment involves usually foliar application till the run-off. This corresponds, in general, with a spray volume of from about 600 to about 1000 liters per hectare of crop locus, depending on the growth stage of the crop. 
     Other pesticides, e.g., fungicides, bactericide, insecticides, acaricides, herbicides or plant growth regulating agents, may be used in addition to cymoxanil and dimethomorph to enhance the activity of the association of the invention or to widen its spectrum of activity. The addition of a contact fungicide is particularly advantageous. 
     The term contact fungicide as used herein is intended to relate to fungicides having no, or no significant, systemic action and comprises, by way of example, copper fungicides; ethylene bis[dithiocarbamato]metal compounds such as mancozeb, maneb, zineb and propineb; captan; captafol; folpet; and chlorothalonil. 
     The weight ratio cymoxanil:dimethomorph will depend on various factors such as the mode of application, the disease to be combatted, the crop involved, the application time, etc. 
     In general, satisfactory results will be obtained when the weight ratio of cymoxanil:dimethomorph lies in the range of from about 1:3 to about 3:1, more preferably from about 2:1 to about 1:2, as illustrated by the experimental test results for the range of from about 1.5:1 to about 1:1.5. 
     The invention also provides fungicidal compositions comprising cymoxanil and dimethomorph, e.g., in a weight ratio within the range specified hereinabove. 
     The method of the invention is particularly useful when a contact fungicide is admixed with cymoxanil and dimethomorph. 
     In general, satisfactory results will be obtained when the weight ratio of contact fungicide:cymoxanil:dimethomorph lies in the range of from about 10:5:1 to 5:1:1, more preferably from about 7.5:1.5:1 to 10:1:1 and most preferably at a ratio of about 5.15:1.5:1, as illustrated by the experimental test results. 
     The invention also provides fungicidal compositions comprising a contact fungicide (e.g., mancozeb, folpet, chlorothalonil), cymoxanil and dimethomorph. 
     Such compositions of the invention may be formulated in any conventional form, for example in the form of a twin packet, triple packet or of an emulsifiable concentrate, a soluble concentrate, a wettable powder or water dispersible granule. Such compositions may be produced in conventional manner, for example by mixing cymoxanil with dimethomorph with appropriate adjuvants such as diluents and optionally other formulating ingredients such as surfactants. 
     The term diluent as used herein means any liquid or solid agriculturally acceptable material, including carriers, which may be added to the active constituents to bring them in a suitable application or commercial form. It can, for example, be talc, kaolin, diatomaceous earth, mineral oil or water. 
     Particular formulations to be applied in spraying forms such as water, dispersible concentrates or wettable powders, may contain surfactants such as wetting and dispersing agents, e.g., the condensation product of formaldehyde with naphthalene sulphonate, an alkylarysulphonate, a lignin sulphonate, a fatty sulphate, an ethoxylated alkylphenol and an ethoxylated fatty alcohol. 
     In general, the formulations include from 0.01 to 90% by weight of active agent, from 0 to 20% of an agriculturally acceptable surfactant and 10 to 99.99% solid or liquid diluent, the active agent consisting of cymoxanil and dimethomorph, with or without a contact fungicide such as mancozeb. The formulations may additionally contain additives such as pigments, thickeners and the like. 
    
    
     The invention is illustrated by the following examples, wherein parts and percentages are by weight. 
     TEST METHODS 
     Determination Of Fungicidal Activity (EC 90) Phytophtora infestans On Potato 
     Potted potato plants (9-12 leaf stage) are sprayed with an aqueous spray liquid comprising 250, 63, 16 and 4 ppm of cymoxanil, dimethomorph, a contact fungicide, a mixture of cymoxanil with dimethomorph in various weight ratios or a mixture of cymoxanil, dimethomorph and a contact fungicide in various weight ratios. The compounds cymoxanil and dimethomorph are employed in aqueous solution of the wettable powder formulation. The mixtures of cymoxanil and dimethomorph, or of both of them together with a contact fungicide, are employed as tank mixtures thereof. 
     One day after the fungicide treatment, the treated plant leaves are inoculated with a sporangium suspension (3×10 3  sporangia/ml) of Phytophthora infestans. Sporangia are taken from inoculated potato tuber slices. The plants are then transferred to a dew chamber providing 100% relative atmospheric humidity at an ambient temperature of 18° C. in the dark. Disease control is evaluated 5-8 days thereafter, by comparing the treated plants (leaves) with untreated, similarly inoculated plants (leaves). 
     The inoculation is performed by using either sensitive fungal strains or strains which developed resistance against phenylamide fungicides. 
     The disease control is expressed for each test concentration as % control. This allows the determination of the EC 90 exp. value using probit analysis, i.e., the concentration of each compound or combination allowing 90% disease control. 
     The experimental results (EC 90 exp.) for a given weight ratio cymoxanil:dimethomorph is compared with the corresponding EC 90 theor. value, i.e., the concentration of that particular mixture allowing 90% disease control calculated according to Wadley. ##EQU1## wherein a and b are the ratios of cymoxanil and dimethomorph in the mixture, respectively, and the indexes (I), (II) and (I+II) refer to cymoxanil, dimethomorph and the a+b mixture of cymoxanil and dimethomorph. In the case of synergism, EC(I+II)90 theor. is greater than EC(I+II)90 exp., or 
     
         SF=[EC(I+II)90 theor.]/[EC(I+II)90 exp.]&gt;1 
    
     The experimental results (EC 90 exp.) for a given weight ratio of a contact fungicide:cymoxanil:dimethomorph is compared with the corresponding EC 90 theor. value, i.e., the concentration of that particular triple mixture allowing 90% disease control calculated according to Wadley. ##EQU2## wherein a, b and c are the ratios of cymoxanil, dimethomorph and the contact fungicide in the mixture, respectively, and the indexes I, II, and III refer to cymoxanil, dimethomorph, the contact fungicide and the a+b+c mixture of the three components, respectively. In the case of synergism, EC(I+II+III)90 theor. is greater than EC(I+II+III)90 exp., or 
     
         SF=[EC(I+II+III)90 theor.]/[EC(I+II+III)90 exp.]&gt;1 
    
     TEST METHODS 
     Determination Of Fungicidal Activity (EC 90) Pseuduperonospora cubensis On Cucumber 
     Young potted cucumber plants (2 leaf stage) were sprayed with fungicides as described above for potatoes. Inoculation was done one day after the treatment with sporangium suspension (1.5×10 3  /ml) of Pseudoperonospora cubensis. Inoculation, disease assessment and calculation of EC 90 were conducted as described for P. infestans in potato. 
     TEST RESULTS 
     The fungicidal activity obtained with cymoxanil (in aqueous spray liquid form of a 50% wettable powder formulation; dimethomorph (in aqueous spray liquid form of a 25% wettable powder formulation; cymoxanil + dimethomorph mixtures in weight ratio 2:1, 1.5:1, 1:1, 1:1.5 and 1:2, against a Phytophthora infestans is expressed in Table I. The EC 90 exp. values for cymoxanil and dimethomorph specified in Table I have been employed to calculate the EC 90 theor. values for the cymoxanil/dimethomorph mixtures expressed in Table I. 
     
                       TABLE I                                                     
______________________________________                                    
P. infestans in potato                                                    
                         EC 90   EC 90                                    
Concentration                                                             
           % Control     exp.    theor.                                   
ppm (a.i.) 250    63     16  4   (ppm) (ppm)  SF                          
______________________________________                                    
Cymoxanil  97     93     84  51  104   --     --                          
Dimethomorph                                                              
           100    97     87  63  30    --     --                          
Cymoxanil to                                                              
Dimethomorph                                                              
Weight Ratio                                                              
1:1        97     97     84  53  96    48     0.5                         
1.5:1      100    100    87  71  15    53     3.5                         
2:1        97     95     60  53  108   54     0.5                         
1:1.5      97     97     73  53  100   40     0.4                         
1:2        68     57     51  51  435   44     0.1                         
______________________________________                                    
 
    
     The fungicidal activity obtained with cymoxanil (as above), dimethomorph (as above), the contact fungicide mancozeb (in aqueous spray liquid of 80% wettable powder), cymoxanil + dimethomorph + mancozeb in weight ratios of 10.5:1:1, 7.5:1.5:1, 5.15:1.5:1 against Phytophthora infestans is expressed in Table II. The EC 90 exp. values for cymoxanil, dimethomorph and mancozeb specified in Table II have been employed to calculate the EC 90 theor. values for the mixtures expressed in Table II. 
     
                       TABLE II                                                    
______________________________________                                    
P. infestans in potato                                                    
                         EC 90   EC 90                                    
Concentration                                                             
           % Control     exp.    theor.                                   
ppm (a.i.) 250    63     16  4   (ppm) (ppm)  SF                          
______________________________________                                    
Mancozeb   45     27     20  13  550   --     --                          
Cymoxanil  97     93     84  51  104   --     --                          
Dimethomorph                                                              
           100    97     87  63  30    --     --                          
Mancozeb to                                                               
Cymoxanil to                                                              
Dimethomorph                                                              
Weight Ratios:                                                            
10.5:1:1   89     83     63  60  209   209    1.0                         
7.5:1.5:1  100    76     73  57  78    164    2.1                         
5.15:1.5:1 100    100    73  68  20    134    6.7                         
______________________________________                                    
 
    
     The fungicidal activity obtained for the triple mixture mancozeb + cymoxanil + dimethomorph in controlling Pseudoperonospora cubensis in cucumbers is illustrated in Table III. 
     
                       TABLE III                                                   
______________________________________                                    
Pseudoperonospora cubensis on cucumber                                    
                          EC 90   EC 90                                   
Concentration                                                             
           % Control      exp.    theor.                                  
ppm (a.i.) 250    63     16   4   (ppm) (ppm) SF                          
______________________________________                                    
Mancozeb   100    90     61   8   54    --    --                          
Cymoxanil  82     61     23   5   258   --    --                          
Dimethomorph                                                              
           100    100    78   67  18    --    --                          
Mancozeb to                                                               
Cymoxanil to                                                              
Dimethomorph                                                              
Weight Ratios:                                                            
5.15:1.5:1 100    100    100  74  16    53    3.1                         
______________________________________                                    
 
    
     The fungicidal activity of the invented triple mixture of mancozeb + cymoxanil + dimethomorph at a near optimal ratio (5.15:1.5:1) was higher than that of either the commercial dual mixture &#34;Mancur&#34; (mancozeb + cymoxanil) or the commercial dual mixture &#34;Acrobat&#34; (mancozeb + dimethomorph). This higher activity is illustrated in Table IV for the control of Phytophthora infestans in potato. 
     
                       TABLE IV                                                    
______________________________________                                    
Phytophtora infestans in potato                                           
                                EC 90                                     
Concentration    % Control      exp.                                      
ppm (a.i.)       250    63      16  4   (ppm)                             
______________________________________                                    
Mancozeb + Cymoxanil 4:1                                                  
                 100    95      85  40  37                                
Mancozeb + Dimethomorph                                                   
                 100    90      44  40  58                                
6.7:1                                                                     
Mancozeb to                                                               
Cymoxanil to                                                              
Dimethomorph                                                              
Weight Ratios:                                                            
5.15:1.5:1       100    100     90  86  13                                
11:1:1           98     95      85  63  82                                
16:2.4:1         100    95      75  75  40                                
______________________________________                                    
 
    
     As can be seen, the best EC 90 was obtained with a ratio of 5.15:1.5:1. 
     The test results presented in Table V demonstrate a synergistic effect for the triple mixture between mancozeb + cymoxanil + dimethomorph in controlling P. Infestans in potted potato plants incubated for 5 days outdoors after fungicidal spray. 
     
                       TABLE V                                                     
______________________________________                                    
Phytophtora infestans in potato                                           
                         EC 90   EC 90                                    
Concentration                                                             
           % Control     exp.    theor.                                   
ppm (a.i.) 250    63     16  4   (ppm) (ppm)  SF                          
______________________________________                                    
Mancozeb   74     65     58  31  336   --     --                          
Cymoxanil  65     39     10  0   343   --     --                          
Dimethomorph                                                              
           96     80     66  28  144   --     --                          
Mancozeb to                                                               
Cymoxanil to                                                              
Dimethomorph                                                              
Weight Ratio:                                                             
5.15:1.5:1 100    82     77  56  65    286    4.4                         
______________________________________                                    
 
    
     The test results presented in Table VI demonstrate the efficacy of triple mixtures composed of mancozeb + cymoxanil + dimethomorph applied once in 2 weeks in controlling P. infestans under field conditions. 
     
                       TABLE VI                                                    
______________________________________                                    
Phytophtora infestans in potato                                           
Fungicide mixture                                                         
             Number of leaflets infected per plot at                      
a.i./ha in g and weight                                                   
             various times after inoculation                              
ratio        at 2 weeks                                                   
                       at 3 weeks at 3.5 weeks                            
______________________________________                                    
None         58 ± 20*                                                  
                       2050 ± 230*                                     
                                  2225 ± 228*                          
Metalaxyl +  0         21 ± 11*                                        
                                  43 ± 19*                             
mancozeb                                                                  
1905 g                                                                    
1:7.5                                                                     
Ofurace + folpet                                                          
             0         103 ± 20*                                       
                                  220 ± 84*                            
2050 g                                                                    
1:7.5                                                                     
Dimethomorph +                                                            
             0         2 ± 1** 4 ± 2**                              
cymoxanil +                                                               
mancozeb                                                                  
2190 g                                                                    
1:1.5:5.15                                                                
2238 g       0         3 ± 1** 8 ± 4**                              
1:1.5:7.5                                                                 
______________________________________                                    
 Note: The first spray was given 1 day before inoculation while the second
 spray was given 14 days after inoculation.                               
 *sporulating lesions                                                     
 **nonsporulating lesions                                                 
 
    
     FORMULATION EXAMPLE 
     
         ______________________________________                                    
        15%  cymoxanil                                                    
        10.% dimethomorph                                                 
        5%   silicagel                                                    
        62%  kaolin                                                       
        5%   sodium ligninsulphonate                                      
        3%   sodium dodecylsulphate                                       
______________________________________                                    
 
    
     FORMULATION EXAMPLE 
     
         ______________________________________                                    
       16.8% mancozeb                                                     
       4.9%  cymoxanil                                                    
       3.3%  dimethomorph                                                 
       5%    silicagel                                                    
       62%   kaolin                                                       
       5%    sodium ligninsulphonate                                      
       3%    sodium dodecylsulphae                                        
______________________________________                                    
 
    
     FORMULATION EXAMPLE 
     
         ______________________________________                                    
       18.75%                                                             
             mancozeb                                                     
       3.75% cymoxanil                                                    
       2.5%  dimethomorph                                                 
       5%    silicagel                                                    
       62%   kaolin                                                       
       5%    sodium ligninsulphonate                                      
       3%    sodium dodecylsulphae                                        
______________________________________                                    
 
    
     The wettable powders of Formulation Examples 1 to 3 are obtained by mixing of the components, subsequent milling of the mixture in an appropriate mill allowing a sufficiently fine particle size (equal to or smaller than 20 micrometer) followed by mixing of the milled material. 
     While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made.