Fitness of Metalaxyl-Sensitive and Metalaxyl-Resistant Isolates ofPhytophthora infestanson Susceptible and Resistant Potato Cultivars

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Abstract
Intact leaflets and tuber disks of 14 cultivars of potato were inoculated with three metalaxyl-sensitive (MS) and three metalaxyl-resistant (MR) isolates of Phytophthora infestans in the absence of metalaxyl. Infection frequency, lesion area, sporulation capacity in leaflets, and sporulation capacity in tuber disks were recorded at 4, 6, 7, and 7 days after inoculation, respectively. Infection frequency varied among cultivars and among fungal isolates. Cara was significantly (P < 0.05) less susceptible to leaflet infection induced by either metalaxyl-sensitive (MS) or metalaxyl-resistant (MR) isolates compared with the other cultivars tested. Lesion area varied significantly among cultivars and between MS and MR isolates. MR isolates produced significantly (P < 0.05) larger lesions than MS isolates in nine cultivars. Lesions were restricted in the other five cultivars (Draga, Cara, Dunja, Tarzan, and Timata) regardless of the isolates used for inoculation. Sporalation in leaflets varied significantly among cultivars and in two cultivars among fungal isolates. Dunja, Tarzan, and Timata allowed for no fungal sporulation in leaflets, whereas in the other 11 cultivars sporulation ranged between about 100 and 26,000 sporangia per square centrimeter of leaflet lesions. In Spunta and Atica, MS isolates sporulated significantly better (P < 0.05) than MR isolates. Sporulation on tuber disks varied among cultivars and among fungal isolates. It was lowest in Avondale, Cara, and Timata and highest in Tarzan. Tuber disks of four cultivars (Atica, Liseta, Cara, and Dunja) supported significantly higher sporulation of MS compared to MR isolates of the fungus. Sporulation in leaflets and tuber disks was poorly correlated, while infection frequency was correlated (r = 0.56) with sporulation in leaflets. The MR group of isolates had a significantly higher compound fitness index compared with the MS groups of isolates in foliage of the potato cultivars tested. This higher fitness, however, did not render the foliage resistance of some cultivars ineffective.