CHEMOTHERAPEUTIC AND BIOCHEMICAL ACTIVITIES OF SOME FUNGICIDES AGAINST SQUASH DOWNY MILDEW DISEASE

The potency of five selected fungicides as chemotherapeutic agents against
squash downy mildew was studied as well as their effect on biochemical changes of
the host plant as chlorophyll, p-carotene and sugar contents, in addition to polyphenol
oxidase activity. The fungicides testecd were propamocarb hydrochloride (Previcur-N),
mikal, mancozeb (Diathane M-45), Ridomil-Plus (metalaxyl-plus) and tolclofos-methyl
(Rizolex). The results revealed that: 1) Propamocarb hydrochloride and mikal
(systemic fungicides) were highly effective in reducing Pseudoperonospora cubensis
infection indicated by low disease index and high recovery of infected squash leaves.
In contrast, non-systematic fungicides as tolclofos-methyl and mancozeb were lowest
affective in this respect. 2) P .. cubensis infection caused significant decrease in
chlorophyll content, p-carotene content and polyphenol oxidase activity of squash
leaves. Sugar content was insensitive toward downy mildew causal pathogen
infection. 3) Fungicide treatments alone showed a feeble effect against chlorophyll
content, p-carotene contents and polyphenol oxidase activity of squash leaves, while
sugar content was increased Significantly especially due to mikal treatments. 4)
Combinations of downy mildew infection and fungicide treatments was in favor of
producing healthy plants and increasing some biochemical components related to the
resistance phenomenon as sugar content and polypenol oxidase activity of squash
leaves. The pigment contents represented as chlorophyll and p-carotene contents,
were increased slightly due to therapeutic effect of fungicides, however, the increasing
value was not able to restore the values of non-infected plants especially with p-
carotene content.