ACTION OF FUNGICIDES 



257 



little used. Formaldehyde reacts with free amino groups, and it is 

 probable that its fungicidal action depends upon this property. Some 

 other aldehydes also have fungicidal properties (Uppal, 1926). 



Quinones. While there are two series of quinones (ortho, or 1,2, and 

 para, or 1,4), we shall consider only the 1,4-quinones as fungicides. The 

 simplest quinone is p-benzoquinone. Quinones are cyclic compounds 

 which possess a characteristic pair of double bonds. Such a configuration 

 of double bonds is called quinoid and is possessed by many dyes, some 

 of which are fungicides. If a considerable series of toxic compounds 

 possess a common functional group or groups, it may be assumed that 

 these groups are involved in fungicidal activity. According to Horsfall 

 (1945), 1,4-benzoquinone has a slight toxicity to fungi. The four hydro- 

 gens in 1,4-benzoquinone can be replaced by chlorine to form chloranil 

 (Spergon), which greatly increases the fungicidal properties. The struc- 

 tural formulas for these compounds are given below: 



O O 



— H 



-H 



CI— 



Cl- 



— CI 



-CI 



o o 



1,4-Benzoquinone Chloranil (Spergon) 



Spergon has been used as a seed protectant. 



Substituted naphthoquinones are more important fungicides than, 

 the benzoquinones. Among these, 2,3-dichloro-l,4-naphthoquinone 

 (Phygon) is reported to be five to eight times as effective as Spergon (Ter 

 Horst and Felix, 1943). Some of the naphthoquinones synthesized by 

 plants are fungicides. Juglone, 5-hydroxy-l,4-naphthoquinone, is found 

 in walnut hulls and is secreted by walnut roots. The isomeric 2-hydroxy- 

 1,4-naphthoquinone (lawsone) is found in henna leaves. Juglone is 

 reported to be as toxic to fungus spores as Bordeaux mixture. Juglone 

 controls black spot of roses as well as sulfur does (Gries, 1943, 1943a). 

 It is also toxic to many plants. Little et al. (1948) isolated 2-methoxy- 

 1,4-naphthoquinone from Impatiens balsamina. This compound was an 

 active fungicide which exhibited no phytotoxicity toward tomato and 

 bean plants. The formulas of two naphthoquinone fungicides are given 

 below: 



O O 



-OCH3 



o 



2-Methoxy-l,4-naphthoquinone 



OH O 



Juglone 



