Incidence and Survival of Cylindrocladium parasiticum in Peanut Seed.

Sixty-three commercial seed lots of peanut produced in Virginia were examined for the presence of seed with speckled testae. Speckled seed were present in seed lots from the 1998, 1999, and 2000 growing seasons at average rates of 3, 1.2, and 0.6%, respectively. Speckled and normal seed from 19 seed lots were assayed on a medium selective for C. parasiticum. The fungus was isolated from speckled seed at rates ranging from 40 to 96%. C. parasiticum was isolated only from a single normal seed from one seed lot. The pathogen was recovered at high rates from speckled seed immediately after pods had been dried in commercial drying trailers at temperatures up to 35°C. Ambient temperatures during winter seed storage that fluctuated from -10 to 28°C in 1999 and -8 to 33°C in 2000 greatly reduced pathogen recovery in speckled seed stored for 16 or 24 weeks. In field plots with naturally infested soil, the number of speckled seed harvested was directly correlated to the number of symptomatic plants in plots on 29 September. Based on this finding, the harvest of seed peanuts in areas of a field with high incidence of Cylindrocladium black rot (CBR) should be avoided. Adoption of this policy is expected to lower the number of speckled seed entering commercial seed lots and reduce the risk for spread of CBR.

An Algorithm for Predicting Outbreaks of Sclerotinia Blight of Peanut and Improving the Timing of Fungicide Sprays.

Algorithms were evaluated for computing disease risk and improving the timing of fungicide applications for control of Sclerotinia blight of peanut. Disease risk was calculated by multiplying indices of moisture, soil temperature, vine growth, and canopy density each day, and summing values for the previous 5 days to obtain a 5-day risk index (FDI). After fungicide application, the FDI was reset to zero for 3 weeks. Fluazinam at 0.58 kg a.i./ha applied at FDI 24 or 32 in 1994 and 1995 suppressed disease and increased yield as much as or more than programs of weekly scouting and applying fungicide at the initial onset of disease with additional sprays at 3- to 4-week intervals. The FDI algorithm was also more efficient than calendar sprays at 60, 90, and 120 days after planting (DAP). Environmental and host parameters were expanded in 1996 and 1997 by adding new temperature and new vine growth indices. These parameters along with DAP-dependent thresholds consistently improved the timing of fungicide sprays and disease management when using the FDI algorithm in comparison to weekly scouting or calendar sprays at 60, 90, and 120 DAP.

First Report of Mimosa Wilt Disease of Silk Tree (Albizia julibrissin) in California Caused by Fusarium oxysporum f. sp. Perniciosum.

A Fusarium sp. was isolated from a 12-year-old Silk Tree (Albizia julibrissin) in a residential area of Redlands, CA. The scaffold branches and trunk exhibited gummosis, the sap oozing from fissures or intact bark. Internally the wood exhibited brown to black broad streaks of discoloration from the scaffold branches down into lateral roots below the root crown, similar to symptoms observed in Virginia (2). Wilted and dried foliage remained on the scaffold branches. Two-week-old cultures of the isolate grown on Komada (1) and acidified potato dextrose agar media developed short conidiophores, macroconidia, and colony morphology typical of Fusarium oxysporum. To complete Koch's postulates, 1-month-old seedlings were root-dip inoculated with a water suspension of macro- and microconidia (106 per ml). Two weeks after inoculation, typical Fusarium wilt symptoms developed in all inoculated seedlings. The fungus was reisolated from symptomatic seedlings. This is the first report of mimosa wilt disease in California. The disease has the potential to adversely impact California's nursery and landscape industry. References: (1) H. Komada. Rev. Plant Prot. Res. 8:114, 1975. (2) R. J. Stipes and P. M. Phipps. Phytopathology 65:188, 1975.