ミカン果実を侵かすPhytophthora citrophthoraおよびPenicillium digitatumの示す病徴はきわめて対照的である。前者が果実を褐変せしめるが軟腐を起さないのに比して後者は変色はほとんど起らずいちじるしく軟腐せしめる。本研究はこの病徴に相違を来たす原因を明らかにするために行なったものである。P. citrophthoraは同じPhytophthora属でも, P. capsiciに較べて軟化酵素の活性は低く, またPen. digitatumよりも低い。両菌が寄主に侵入するとともに罹病部pHが低下し始め, 2-3日後にP. citrophthoraの軟化酵素の最適pH域の外に出てしまう。しかしPen. digitatumの軟化酵素の最適pH域は低いところにあるため軟化はさらに進行する。またミカン果皮中にP. citrophthoraの軟化酵素活性をいちじるしく阻害する物質が含まれているのと相まってミカン果実でP. citrophthoraによる軟化は進行しないものと考えられる。Pen. digitatumの軟化酵素はendopolygalacturonase (endo-PG)を, P. citrophthoraはendo-PGとともにexo-polygalacturonase (exo-PG)も分泌しているようである。pectin estrase (PE)およびtrans-eliminase (TE)活性は両菌ともにほとんどみとめられない。褐変原因と考えられるphenol類は果皮中にはchlorogenic acidのみしかみとめられなかったが, これは両菌の発育ならびにPG分泌に対してほとんど阻害を示さないようであった。解剖所見によっても褐変部位を越えてP. citrophthoraの菌糸が伸展していることが見られる。Among the Citrus diseases, the symptoms of the brown rot of fruit caused by Phytophthora citrophthora show remarkable contrast to that of the common green mold disease by Penicillium digitatum. The former fungus changed the host tissue to brown colour but didn\u27t macerate it, the latter didn\u27t change colour although marked maceration progressed. The difference of the mechanisms on the reaction of Citrus fruits against these two fungal infections were studied in this paper. To evaluate the macerating activity the discs of cucumber fruit and potato tuber were used. The macerating activity of the host-pathogen complex in the lesion reached a maximal peak at 3rd day after inoculation with P. citrophthora, but fell down during following several days. With Pen. digitatum, the maximal activity was continued over several days after that. Optimum pH range for the maceration was in 5-6 for Pen. digitatum and in 3-4 for P. citrophthora respectively and the activity of the former was higher than the latter. On the other hand, pH of the lesion formed by each pathogen began to drop from 3rd day after inoculation and then reached a stationary value, 3-4,at 5th day. In this pH range the macerating enzyme secreted from P. citrophthora didn\u27t show their activity, moreover the activity was inhibited by the extracts from the pericarp of Citrus fruit. The facts described above can explain some of the reasons by which soft rot didn\u27t developed in the case of the brown rot by P. citrophthora. Pen. digitatum have endo-PG, and P. citrophthora have endo- and exo-PG, but both fungi didn\u27t secret trans-eliminase
