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Single plant yield loss due to brown girdling root rot (Rhizoctonia solani) and cabbage root maggot (Delia spp.) damage was assessed in canola (Brassica rapa cvs. Reward and Tobin) to quantify the effects of root rot on yield and determine whether yield losses due to root rot are compounded by maggot damage. At crop ripening during 1998 and 1999, 2,000 plants from six growers' fields were scored for both root rot and maggot injury. The assessed roots were separated into eight categories constructed by crossing four classes of root rot severity based on degree of girdling and two classes of maggot damage based on percentage of the root surface with maggot tunnels. A ninth category was defined for partially to completely decayed roots for which it was not possible to determine maggot damage. There was no root rot by maggot injury interaction on any of the yield parameters measured. Furthermore, maggots had no adverse effects on yield at the observed damage levels. In contrast, plant weight, seed yield, harvest index, seed size, but not oil content were reduced on plants with completely decayed roots. On average, seed yield was not reduced on plants with roots with nongirdling lesions and superficial nonsinking girdling lesions, but was reduced by 17% on plants with roots with girdling and sinking lesions, and by 65% on plants with decayed taproots. Over 2 years, yield losses in growers' fields ranged from 1 to 5%.

Brown girdling root rot (BGRR) is a serious and widespread disease of canola (Brassica rapa L.) in the Peace River region of northwestern Canada. There is no chemical control treatment for the pathogen, and farmers have observed that the disease is more severe when canola follows red fescue (Festuca rubra L.) or clover (Trifolium spp.) compared to summer fallow. A field study was conducted to determine how crop sequences following red fescue termination can be combined with residue and tillage management to reduce BGRR infection and increase canola yield. The five treatments consisted of rotations of: continuous canola (CCC) and oat (Avena sativa L.)-oat-canola (OOC), both managed using reduced tillage (RT), and wheat (Triticum aestivum L.)-wheat-canola (WWC), managed using RT, conventional tillage (CT) or no-till (NT). Canola yield followed the trend: OOC (RT) = WWC(RT) > WWC(CT) > CCC(RT) = WWC(NT). BGRR infection increased with tillage intensity: WWC(CT) > CCC(RT) = WWC(RT) = OOC(RT) > WWC(NT), and was reduced when canola followed two cereal break crops. Yield was highest when canola was preceded by a cereal crop and lowest without a break crop. The low yield with NT was attributed to poor crop emergence from a hard seed bed with unbroken turf and to competition from re-emerged fescue in the third year after fescue breaking. This study demonstrated that the cropping sequence and tillage system used influenced canola yield to a greater extent than did BGRR infection. Key words: Brown girdling root rot, canola, crop sequence, red fescue termination, tillage

Brown girdling root rot is a devastating disease of canola (Brassica rapa) in the Peace river region. Breeding for resistance to this disease up to now has relied on field screening, with every alternate row as a check. Examination of the distribution of brown girdling root rot from spaced rows of Brassica rapa ‘Tobin’ revealed that there would be a minimal loss of precision in breeding programs by reducing the frequency of check rows to one in four, which represents a considerable saving in cost compared with the current practice. Key words: Turnip rape, Brassica rapa, brown girdling root rot, resistance, selection technique

S-alleles W1 and T2 and an incompletely dominant white petal character were introgressed into the self-compatible (SC) summer rape (Brassica napus L. ssp. oleifera {Metzg.}) cultivars Global, Topas and Westar. The derived self-incompatible (SI) lines were evaluated for strength of incompatibility by ultraviolet fluorescence of pollen tubes, and by seed set. Pollen tube and seed set analyses showed the W1 and T2 alleles were strongly, moderately and weakly expressed in Topas, Global and Westar, respectively. Seed set data showed a significant difference between SI lines, but not between S-alleles, or between homozygous or heterozygous lines from the same SI cultivar. SI cultivar yellow petal (wild type) lines were field pollinated with SC white petal lines. Seed collected from the SI cultivars were evaluated for proportion of outcrossed progeny by recording the frequency of yellow petal and cream petal plants, which were the result of self- and cross-pollination, respectively. The proportion of outcrossed progeny (i.e., outcrossing rates) ranged from 23% to 79%. Topas SI lines had significantly higher outcrossing rates than Global SI lines, which corresponded to SI line seed set data. Environment, S-allele selection and genotype significantly affected outcrossing rates. Key words: Brassica napus, sporophytic incompatibility, S-allele, outcrossing rate

Two glasshouse experiments were conducted to determine the potential for Armillaria species to infect fireweed and subsequently use it as a food base for attacking lodgepole pine. Aspen segments colonized by A. mellea and A. ostoyae were used to inoculate fireweed roots. Both species caused infection, although the former was more pathogenic, attacking 28 of 42 inoculated fireweed roots as opposed to 8 of 60 in the case of the latter species. None of the nine pines inoculated with segments of fireweed roots colonized by A. ostoyae became infected, even though the isolates were pathogenic on pine when aspen segments were used as the food base. In contrast, 8 of 29 pine seedlings inoculated with fireweed colonized by A. mellea were killed, thus suggesting that fireweed could play some role in the epidemiology of this disease.

Small patches of dead trees were common among 7- to 15-year-old Pinns contorta var. latifolia in Alberta, Canada. The sizes of clones of Armillaria ostoyae, determined by pairing diploid isolates, was larger than these patches. Pairing diploid isolates on strips of Betula papyrifera provided a clear indication of clonal relationships.

Juvenile lodgepole pine (Pinuscontorta Dougl. var. latifolia Engelm.) were excavated at three sites near Hinton, Alberta. In all cases in which Armillariaostoyae (Romagn.) Herink had become established in the root collar or taproot, it was also able to colonize lateral roots. In cases in which only lateral roots were infected, subsequent colonization generally was primarily distal to the point of infection. Rhizomorphs were associated with 89% of 21 infected roots, whereas only 19% of 70 roots with no associated rhizomorphs were infected. Stumps, roots, and debris from the previous generation of trees were the inoculum sources for 78% of 36 infected juvenile trees, and infected regeneration served as the inoculum source for the remaining trees. Rhizomorphs occasionally were attached to the roots or rhizomes of plants other than pine, especially fireweed (Epilobiumangustifolium L.). There was no spatial relationship between stumps and symptomatic trees. Nearest neighbor analysis indicated that the likelihood of an individual tree developing symptoms was dependent on whether trees within 0.15 m were dead or dying but independent of the apparent health of trees at greater distances.

Yield gaps, and factors causing them, are of prime importance for agricultural production. There are too many yield-constraining factors in the variable rainfed lowland rice agroecosystems to be studied in full factorial experiments. An empirical-exploratory attempt is presented to characterize yield gaps and attainable yield levels as related to agroecosystem variables and to adjust yield expectations and identify research and management needs. Data on crop and pest management practices, soil conditions, weather, crop performance, and biotic and abiotic stresses were collected in over 600 plots in farmers’ rainfed lowland ricefields in northwest Luzon (Philippines), northeast Thailand, and the Mekeong River delta (south Vietnam) from 1992 to 1994. The CERES-Rice simulation model was used to estimate weather and nitrogen (N) limited attainable yield levels, while a simple empirical model was used to estimate yield trends based on fertilizer N and soil organic matter. Results of simulation runs indicated that the weather-adjusted yield gaps, i.e. the deviations of the observed yields from the weather-limited simulated yields, averaged about 35% in the Philippines, 45% in Vietnam, and 55% in Thailand. They were mainly due to N limitation in Thailand, where soil-N and fertilizer use is low, while in the other two countries they were mainly related to other constraints. In multiple regression analyses, terms related to soil carbon content and/or amount of N fertilizer captured the main yield trend well. The remaining yield variation could partly be explained by interactions between these main terms and severity levels of diseases and pest damage, water stress, and other variables related to soil conditions, crop and pest management, and weather. Factor and canonical correspondence analyses gave further insights into links among actual yield, estimated attainable yield levels and corresponding yield gaps, and other agroecosystem variables. The approach is useful for quantifying attainable yield levels and yield gaps at various constraint levels and leads to a better understanding of complex relationships between agroecosystem variables, improved yield expectations, and a better understanding of the role of yield-determining factors. This can be instrumental for prioritizing research on yieldlimiting factors and guiding crop and pest management decisions.

In three field trials on the epidemiology of blast disease in upland rice, different methods of sampling spores of Pyricularia grisea were compared to study aerial concentration, deposition, survival, and infectiousness of conidia of the pathogen. The methods included a Burkard spore trap, glass slides and rods, several trap-plant treatments, and leaf prints from trap plants. The results were mostly highly intercorrelated and revealed a unimodal pattern of disease progression, with peaks before or at the middle of the cropping seasons. The number of conidia per square centimeter deposited on leaf surfaces as measured by leaf prints made up one-fourth of the number caught with glass slides [...]