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Notes: In a controlled environment (15/10°C) (day/night) container experiment on winter wheat (cv. Avalon), eyespot incidence (percentage of plants affected) and number of leaf sheaths penetrated after 6 weeks increased with inoculum concentration (102−106 conidia mL−1) of Oculimacula yallundae (OY) or Oculimacula acuformis (OA), but there was no difference between the two species. In an outdoor container experiment, seedlings inoculated with OY 2 weeks after sowing had a greater incidence of eyespot than those inoculated with OA, when assessed 7 weeks after inoculation. Seedlings inoculated with OA at 10 or 20 weeks after sowing developed more severe eyespot by maturity than those inoculated with OY. In an experiment at 15/10°C with seedlings inoculated with OY + OA 2 weeks after sowing, more leaf sheaths were penetrated by OY (3·0 per plant) than OA (2·3 per plant) 6 weeks after inoculation. Field experiments with winter wheat consistently showed leaf sheath production, leaf sheath death, and number of leaf sheaths infected or penetrated by OA or OY were related linearly to thermal time (°C days) after sowing. Depending on cultivar, season and sample, a new leaf sheath was produced in 116–216°C days; a leaf sheath died in 221–350°C days; and infection of a new leaf sheath occurred in 129–389°C days. The mean number of living leaf sheaths infected differed between samples, cultivars and seasons for both OY and OA. Regression analysis of the 1985/86 data suggested that OY progressed more rapidly than OA through the leaf sheaths, and that both the pathogens progressed more rapidly than the rate of leaf sheath death, but more slowly than the rate at which leaf sheaths were produced. It also suggested that OA progressed more slowly than the rate at which leaf sheaths died in 1987/88, but OY did not.

Notes: Factors affecting the production of conidia of Peronosclerospora sorghi, causing sorghum downy mildew (SDM), were investigated during 1993 and 1994 in Zimbabwe. In the field conidia were detected on nights when the minimum temperature was in the range 10–19°C. On 73% of nights when conidia were detected rain had fallen within the previous 72 h and on 64% of nights wind speed was 〈 2.0 m s−1. The time period over which conidia were detected was 2–9 h. Using incubated leaf material, conidia were produced in the temperature range 10–26°C. Local lesions and systemically infected leaf material produced 2.4–5.7 × 103 conidia per cm2. Under controlled conditions conidia were released from conidiophores for 2.5 h after maturation and were shown to be well adapted to wind dispersal, having a settling velocity of 1.5 × 10−4 m s−1. Conditions that are suitable for conidia production occur in Zimbabwe and other semi-arid regions of southern Africa during the cropping season.

Notes: Airborne propagules of Aspergillus flavus were quantified to investigate population dynamics of A. flavus in a region of south-west Arizona prone to epidemics of aflatoxin contamination of cottonseed. Air was sampled continuously from May 1997 to March 1999 at two sites using Burkard cyclone samplers. One sampler was initially at the centre of 65 ha of cotton treated with an atoxigenic strain of A. flavus to manage aflatoxin contamination of cottonseed. The second sampler was 0·6 km from the treated field. Total fungal colony-forming units (CFU) sampled ranged from 17 to 667 and from 9 to 1277 m−3 at the untreated and treated sites, respectively. Counts of A. flavus ranged from 0 to 406 m−3 and from 0 to 416 per m−3 at the untreated and treated sites, respectively. Aspergillus flavus comprised 1–46 and 1–51% of the total cultured fungi at the treated and untreated sites, respectively. Peaks in total fungal and A. flavus CFU coincided with boll maturation and cotton harvest (days 251–321). Autoregression analysis suggested that there was no difference in total fungal CFU between treated and untreated sites, but the analysis showed that the quantity of A. flavus decreased at the treated site. This is probably caused by changes in cropping making the conditions less conducive to growth and reproduction of A. flavus in the surrounding fields. The incidence of the S strain of A. flavus was highest between May and August. The L strain accounted for up to 100% of the A. flavus sampled in the other months, and autoregression analysis showed that the L strain accounted for a greater overall proportion of the A. flavus population at the treated site compared with the untreated site. Autoregression analysis also showed the vegetative compatibility group of the applied strain was a greater proportion of L-strain A. flavus at the treated site (5–75%) than at the untreated site (0–65%), although this decreased with time. The quantity of A. flavus sampled at both treated and untreated sites was correlated with air and soil temperature. Large quantities of A. flavus occurred in the soil (up to 34 474 CFU g−1) of cotton fields and on cotton plant parts and debris (up to 272 461 CFU g−1) adjacent to the cyclone samplers. Aspergillus flavus is a major constituent of the airborne mycoflora associated with cotton fields in south-west Arizona when temperature is conducive to fungal growth. Although application of atoxigenic A. flavus altered the proportion of A. flavus strains and vegetative compatibility groups in the aerial mycoflora, the total quantity of A. flavus remained similar to that in untreated fields. Dispersal of A. flavus between fields suggests that atoxigenic fungi will be most effective in area-wide management programmes.

Notes: The effect of wind on the dispersal of oospores of Peronosclerospora sorghi, cause of sorghum downy mildew (SDM) is described. The oospores are produced within the leaves of aging, systemically infected sorghum plants. These leaves typically undergo shredding, releasing oospores into the air. Oospores are produced in large numbers (6.1 × 103 cm−2 of systemically infected leaf) and an estimate of the settling velocity of single oospores (0.0437 m s−1) of P. sorghi indicated their suitability for wind dispersal. In wind tunnel studies wind speeds as low as 2 m s−1 dispersed up to 665 oospores per m3 air from a group of leaves previously exposed to wind and displaying symptoms of leaf shredding. The number of oospores dispersed increased exponentially with increasing wind speed. At 6 m s−1, up to 12 890 oospores per m3 air were dispersed. Gusts increased oospore dispersal. A constant wind speed of 3 m s−1 dispersed a mean of 416 oospores per m3. When gusts were applied the mean was 15 592 oospores per m3. In field experiments in Zimbabwe, oospores were sampled downwind from infected plants in the field and at a height of 3.8 m above ground level immediately downwind of an infected crop. These data indicate that wind could play a major role in the dispersal of oospores from infected plants in areas where SDM infects sorghum, perhaps dispersing oospores over long distances.