Figure 2. Evaluation scale for late blight resistance in whole plants.

Figure 1. After inoculation, conditions must remain favorable to late blight development as in this screenhouse where a cool mist is provided by an overhead sprinkler system, controled by a thermostat. The shade cloth hung from the ceiling helps maintain a favorable temperature.

a) Previously propagate the isolate of Phytophthora infestans in potato leaf discs or leaflets to maintain the virulence and aggressiveness characteristics. The pathogen can also be propagated in culture medium (V-8 o Rye B Agar).

b) Plants should be between 45-60 days old and grown under optimal conditons. Neither fungicide nor foliar fertilizer should be used as they could interfere with the development of the pathogen. The plants should be turgent at the time of inoculation. It is advisable to spray the leaves with water to eliminate dust particles, etc.

Inoculation Procedure

a) Prepare the inoculum by washing the inoculated leaf discs or leaflets with distilled water. Filter through membranes of 30 um and 10 um and determine the concentration of sporangia.

b) Inoculate each plant with approximately 15-20 ml of a suspension of 3500-5000 sporangia using a manual aspirator until the inoculum begins to run off the leaves. The concentration of sporangia or zoospores depends on the criteria of each investigator. It is preferable to direct the spray to the underside of the leaves. At least three plants per genotype or variety should be inoculated.

c) Incubate the plants for 6 days in a controled enviroment: temperature 18°C, photoperiod of 12 hours and relative humidity 90%. The plants can be kept in plastic-covered mesh containers, or in the open air in the screenhouse and covered in the evenings to maintain the relative humidity. This method is recommended during cold weather with conditions favorable to the development of the pathogen. (See Figure 1)