Occurrence of Phytophthora Blight on Pumpkin in New Mexico
Authors: Sanogo S. , Clary M. 
Published: May 2007

During 2004 and 2005, two fields in Luna County and one field in Doña Ana County in southern New Mexico displayed pumpkin (Cucurbita pepo cv. Magic Lantern) plants with necrotic spots on leaves and dark brown and water-soaked lesions on petioles, vines, and fruits. Lesions on several fruits were covered with white mycelial mats that were also visible in the lumen when fruits were sectioned. Areas of fields with plants displaying these symptoms ranged from 2 to 5%. Within affected areas, percentage of plants with symptoms ranged from 75 to 100%. Samples of leaves, vines, and fruits were collected for isolation of putative causal agents. In all instances, mycelial colonies emerged from leaf, vine, and fruit tissues placed on water agar and incubated at 23 to 25°C. Colonies were transferred to potato dextrose agar and V8 agar media for identification. Two mycelial isolates yielding cultures with white and stellate mycelium were recovered. Biometric data were collected on 50 sporangia produced using the mycelium-agar-disc-in-water method (1) under continuous fluorescent light for 7 days at room temperature. Sporangia were caducous, ellipsoid, and papillate with an average length of 42.6 μm and average breath of 25.6 μm. Sporangia had long pedicels with an average length of 71.5 μm. Oogonia that formed only when the isolates were paired with an opposing mating type tester of Phytophthora capsici had an average diameter of 29 μm with amphigynous antheridia. Both isolates were of mating type A1. These morphological features and biometric data are consistent with those reported for P. capsici (2). The two recovered isolates of P. capsici were tested in the greenhouse for pathogenicity on the pumpkin cv. Magic Lantern. In each of two trials, 25 plants at the first fully expanded leaf stage were inoculated by dispensing 5 ml of zoospore suspension (2,000 zoospores per ml) of each isolate on top of the soilless mix around the base of each plant. Within 5 to 7 days of inoculation, symptoms were visible as girdling, dark brown lesions on stems. Samples of stems from 10 infected pumpkin plants were plated on water agar. P. capsici was recovered from all sampled plants. Control plants (noninoculated) did not display any symptoms. Pumpkin isolates also were inoculated onto 30 chile pepper (Capsicum annuum) plants of cv. AZ-20, which is susceptible to P. capsici. In each of two trials, 15 chile pepper plants at the 6- to 8-leaf stage were inoculated by the same method used for pumpkin plants. Within 7 to 10 days, dark lesions appeared on stems followed by defoliation and wilting. P. capsici was recovered from sampled stem segments. Control plants (noninoculated) did not display any symptoms. Infection of pumpkin by P. capsici has been reported in several states in the United States such as Illinois, Michigan, and North Carolina (3). However, to our knowledge, this is the first report of P. capsici on pumpkin in New Mexico, and it has implications for chile pepper, a major crop in southern New Mexico known to be susceptible to P. capsici. The first occurrence of P. capsici on chile pepper was reported in 1922 (4). Pumpkin and chile pepper are grown in the same fields. In light of the cross-infectivity of P. capsici to both crops, it is not advisable for growers to continuously rotate pumpkin and chile pepper.

References: (1) S. S. A. Al-Hedaithy and P. H. Tsao. Mycologia 71:392, 1979. (2) D. C. Erwin and O. K Ribeiro. Phytophthora Diseases Worldwide. The American Phytopathological Society. St. Paul, MN. 1996. (3) M. K. Hausbeck and K. H. Lamour. Plant Dis. 88:1292, 2004. (4) L. H. Leonian. Phytopathology 12:401, 1922.