Greenhouse evaluation of chemicals for control of powdery mildews

2,6-Dinitro-4-s-alkylphenols were found to protect apple foliage against powdery mildew more effectively than isomeric 2,4-dinitro-6-s-alkylphenols; regression lines for four para-alkyl compounds were of similar slope but were much steeper than those for two ortho-alkyl compounds (including dinocap phenol). The ED₅₀ and ED₉₅ values of the most active compound studied, 2,6-dinitro-4-(I-ethylhexyl)phenol, were in the ratios to those of dinocap phenol of 1:13 and 1:125, respectively, and the protective action of this compound was greater than the curative, especially at the higher ED values. Protection of barley seedlings against powdery mildew was also greater by I-ethyl- or I-propyl-alkyl compounds than by I-methylalkyl or n-alkyl isomers. For phytotoxic and acaricidal actions, the ortho-alkyl isomers are more effective than para-alkyl. Nevertheless, the acaricidal activity of dinocap phenol is exceeded by that of the isomeric 2,4-dinitro-6-(I-ethylhexyl) phenol. The control of these powdery mildew diseases given by commercial products, supposed to be based on dinocap, cannot be accounted for by the activity shown for dinocap phenol. It can, however, be accounted for by the activities of 2,6-dinitro-4-(I-ethylhexyl)- and -(I-propylpentyl)-phenols, now known to be present in commercial products in larger amounts than dinocap phenol itself. The phytotoxic and acaricidal actions of such products, however, are mainly due to the ortho-octyldinitrophenols present; in view of the small proportion that is dinocap phenol, the acaricidal activity is likely to be due almost entirely to the other ortho-octyl isomers. It is suggested that the common name dinocap be retained for the mixture of dinitrooctylphenols now known to be present in commercial products, and that the two main groups be differentiated as 2,4-dinocap and 2,6-dinocap, respectively. The advantages for powdery mildew control of a product based on 2,6-dinocap are discussed.