Vegetative growth response of round-leaved mallow to various day/night temperature regimes was studied under controlled-environment conditions to predict its potential geographic distribution and to develop an effective control program. Round-leaved mallow dry matter production was greatest with day temperatures of 18 to 26 C. Dry matter accumulation was reduced by a night temperature of 6 C but was minimally affected by night temperatures ranging from 12 to 24 C. Regression analysis predicted minimal vegetative growth at mean daily temperatures below 8 C and above 30 C, with optimum growth at 20 C. Partitioning of round-leaved mallow biomass in leaves, stems, and roots was affected by temperature. Maximum leaf weight ratio occurred at low temperatures, 10 C day and 6 C night. Stem weight ratio was greatest at a day temperature of 26 C, with night temperature having little effect. Maximum root biomass occurred with a day temperature of 18 C. Results are discussed in terms of environmental conditions that allow round-leaved mallow to be an effective competitor with crops and potential approaches for its control.

Colletotrichum gloeosporioides f. sp. malvae (C.g.m.) was evaluated under greenhouse conditions for round-leaved mallow control in combination with the following herbicides: bromoxynil plus MCPA (1:1), diclofop, imazethapyr, metribuzin, sethoxydim, and 2,4-DB. Treatments consisted of C.g.m. and herbicide applied alone, tank mixed, and C.g.m. applied 48 h before, and 1, 24, and 72 h after herbicide. Split applications with bromoxynil plus MCPA, imazethapyr, metribuzin, (C.g.m. before or after) and sethoxydim (C.g.m. before) enhanced round-leaved mallow control over C.g.m. applied alone. However, diclofop or 2,4-DB tank mixed or applied in a split application with C.g.m. did not improve round-leaved mallow control over C.g.m. applied alone. Bromoxynil plus MCPA, metribuzin, 2,4-DB, and imazethapyr may be tank mixed with C.g.m. without significantly reducing round-leaved mallow control, unlike sethoxydim or diclofop which inhibited C.g.m. development. Dew period did not influence the effectiveness of bromoxynil plus MCPA applied 30 h before C.g.m., but round-leaved mallow control increased with increasing dew period for metribuzin applied 30 h before C.g.m.

Latent infections of Colletotrichum gloeosporioides f.sp. malvae (C. g. malvae) in plants and seed were determined in several crops under field conditions after application of C. g. malvae conidia at 12 times the recommended rate. Colletotrichum g. malvae was isolated in trace amounts from Brassica napus/campestris, Linum usitatissimum, Lens culinaris, Brassica juncea/hirta, Helianthus annuus, and Triticum spp. only at the 2-wk isolation. Colletotrichum g. malvae was never isolated from Beta vulgaris. After 5 wk, C. g. malvae was not detected in any of the crops, except for Carthamus tinctorius at harvest, and was not detected from wash water of plant tissue or seed after 72 h. Colletotrichum g. malvae recovery in the field, as in controlled environmental conditions, decreased with time, which indicates that buildup of inoculum did not occur. Fungi other than C. g. malvae and bacteria isolated from seed and from wash water of plant tissue increased with time throughout the growing season. Colletotrichum g. malvae was not recovered from seed or crowns of any of the crop cultivars at either location in either growing season, except for C. tinctorius. Only one seed of the C. tinctorius cultivar ‘S-208’ out of 17,280 C. tinctorius seed analyzed over three growing seasons tested positive for C. g. malvae. Colletotrichum g. malvae was found only on the seed coat; the germinated seedling was not infected. Colletotrichum g. malvae was not recovered from any of the wash water from seed. These results should alleviate any of the concerns regarding the fate of the millions of conidia applied during a normal spray operation on nontarget plants, the potential infectivity of nontarget plants, the potential buildup of C. g. malvae in the field, and the utilization of any of the C. g. malvae-treated crops for food or feed.

Round-leaved mallow interference in spring wheat and flax was investigated under field conditions in southern Manitoba. Additionally, the growth of mallow alone and in a wheat crop was compared. In six of seven trials, wheat yield losses due to mallow interference were not significant. In a seventh trial, where mallow densities of up to 237 plants m−2 were recorded, wheat yields were reduced up to 15%. In contrast, significant flax yield losses occurred in each of three trials where maximum mallow densities ranged from 21 to 52 plants m−2. From the regression equations it was calculated that 20 mallow plants m−2 caused flax yield losses of 33, 9, and 10% in 1987, 1988, and 1989, respectively. Periodic sampling of mallow plants growing alone and in wheat, and regression analysis of these data using a logistic function as a biological model of plant growth indicated that mallow growth was severely suppressed by wheat By the eighth week after emergence, the lamina area and shoot dry matter of mallow plants growing in wheat was less than 3% of plants growing alone. Similarly, in wheat, mallow produced less than 1% of the seed produced by plants growing alone. This extreme suppression of mallow growth corresponded with an 80 to 90% decrease in photosynthetically active radiation penetrating the wheat canopy for approximately six continuous weeks, beginning 4 wk after crop emergence.

The competitive ability of annual round-leaved mallow was determined in spring wheat and lentil at Indian Head and Regina, Saskatchewan, in 1985 and 1986 using paired quadrats. Significant biomass and seed yield loss occurred in three of four tests in lentil and two of three tests in spring wheat. Differences in numbers of wheat tillers produced between weedy and weed-free plots were found in three of four tests. A two-variable model comprised of early season crop density loss and round-leaved mallow biomass best accounted for the majority of variation in crop yield loss for both lentil and wheat, and tiller density loss in wheat. In 1985 at Indian Head, where no yield loss occurred for either wheat or lentil, round-leaved mallow had been seeded immediately before the crop. Greater yield losses occurred at Regina, in the presence of an older, well-established infestation. In the years and locations with the greatest crop yield losses, round-leaved mallow emerged before the crop causing poor crop emergence. At Regina in 1986, crop yield losses were more than 60% in wheat and 90 to 100% in lentil because of large differences in crop density between weed-free and weedy subplots. Round-leaved mallow exhibited great variability in growth, producing more biomass per plant, more capsules per plant, and more capsules per gram of biomass in the less competitive crop, lentil, than in wheat. Density and biomass of round-leaved mallow were not correlated; with a density of 200 plants m−2, round-leaved mallow biomass in wheat ranged from 100 to 500 g m−2; while in lentil, from 200 to as high as 1000 g m−2, approximately double the range found in wheat. The type of round-leaved mallow infestation (newly seeded or well-established) and environmental conditions (mainly early season precipitation) account for differences between sites and between years.