Since 1991, dramatic changes have occurred in the genetic composition of populations of Phytophthora infestans in the United States. Clonal lineages recently introduced into the United States (US-7 and US-8) are more common now than the previously dominant lineage (US-1). To help determine why these changes occurred, four clonal lineages of P. in-festans common during the early 1990s in the United States and Canada were evaluated for sensitivity to the protectant fungicides mancozeb and chlorothalonil using amended agar assays for isolates collected from 1990 to 1994. No isolate or lineage was resistant to either mancozeb or chlorothalonil. There were significant differences among isolates for degree of sensitivity to one fungicide individually, but there were no significant (P = 0.05) differences among the US-1, US-6, US-7, and US-8 clonal lineages for degree of sensitivity to both fungicides. Therefore, resistance to protectant fungicides cannot explain the rapid increase in frequency of the US-7 and US-8 clonal lineages. Three components of pathogenic fitness (latent period, lesion area, and sporulation after 96 h) were tested for the three clonal lineages that were detected most commonly during 1994 (US-1, US-7, and US-8). All but one of the isolates in this analysis were collected during 1994 and evaluated within 10 months of collection by inoculating detached leaflets of the susceptible potato cultivar Norchip. There were significant differences between the US-1 and US-8 clonal lineages for lesion area and sporulation, and between US-1 and US-7 for latent period. The US-6 clonal lineage was excluded from the pathogenic fitness experiments, because no isolates of this lineage were collected during 1994. Compared with US-7 and US-8, US-1 had the longest latent period and the smallest lesions with the least sporulation. Incorporation of the differences between US-1 and US-8 in computer simulation experiments revealed that significantly more protectant fungicide (e.g., 25%) would be required to suppress epidemics caused by the US-8 clonal lineage compared with US-1. These differences in pathogenic fitness components probably contribute to the general predominance of the “new” clonal lineages (especially US-8) relative to the “old” US-1 lineage.

The incidence of begomovirus infections in crop plants sharply increased in Brazil during the 1990s following the introduction of the invasive B biotype of the whitefly vector, Bemisia tabaci . It is believed that this biotype transmitted begomoviruses from noncultivated plants to crop species with greater efficiency than indigenous B. tabaci biotypes. Either through rapid host adaptation or selection pressure in genetically diverse populations of noncultivated hosts, over the past 20 years various previously unknown begomovirus species have became progressively more prevalent in cultivated species such as tomato. Here we assess the genetic structure of begomovirus populations infecting tomatoes and noncultivated hosts in southeastern Brazil. Between 2005 and 2010, we sampled and sequenced 126 DNA-A and 58 DNA-B full-length begomovirus components. We detected nine begomovirus species in tomatoes and eight in the noncultivated host samples, with four species common to both tomatoes and noncultivated hosts. Like many begomoviruses, most species are obvious interspecies recombinants. Furthermore, species identified in tomato have probable parental viruses from noncultivated hosts. While the population structures of five well-sampled viral species all displayed geographical subdivision, a noncultivated host-infecting virus was more genetically variable than the four predominantly tomato-infecting viruses.

The effect of solar irradiance (SI) on the viability of sporangia of isolates belonging to two clonal lineages, US-1 and US-8, of Phytophthora infestans was assessed. Exposure during a 3-h period on sunny days (SI > 600 W/m²) drastically reduced germination regardless of the time of day of the exposure. After 1 h of exposure on sunny days, the viability of sporangia decreased by ≈95%, and the effective time necessary to inactivate 95% of the sporangia was 1.1 h. The effective dose to inactivate 95% of the sporangia on sunny days was 2.6 MJ/m². On overcast (SI < 300 W/m²) days, survival after 3 h was reduced only slightly. Thus, other variables being equal, sporangia will survive hours longer in the atmosphere on cloudy days than on sunny days.

Seven organic and inorganic salts were tested for suppression of silver scurf, a postharvest disease of potato tubers caused by Helminthosporium solani. Potassium sorbate, calcium propionate, sodium carbonate, sodium bicarbonate, potassium carbonate, potassium bicarbonate, and ammonium bicarbonate were added to V8 agar at concentrations of 0.06–0.2 M. Radial growth of H. solani was significantly reduced (P < 0.05) by all salts at all concentrations. All salts except sodium bicarbonate and potassium bicarbonate were fungicidal at 0.2 M. Effects of postharvest salt treatments on disease severity and sporulation of H. solani on inoculated and naturally infected potato tubers were evaluated in separate experiments. Greenhouse-grown tubers were inoculated with H. solani spore suspensions (2 × 10⁵ spores/ml), incubated for 5 days, dipped into 0.2 M solutions of each of the salts, and incubated in a moist chamber at 22–24°C for 6 weeks. All salt treatments prevented lesion development and sporulation of H. solani and differed significantly (P < 0.05) from the control. Postharvest applications of test compounds (0.2M) also significantly (P < 0.05) reduced disease severity and H. solani sporulation on naturally infected, field-grown tubers after 15 weeks of storage at 10°C. Salt treatments did not differ in disease or pathogen suppression. These relatively nontoxic organic and inorganic salts have potential as postharvest applications for control of silver scurf.