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Douglas V. Shaw and Kirk D. Larson

Performance characteristics for eighteen strawberry cultivars (Fragaria ×ananassa), nine from California and nine from other North American sources, were evaluated in annual hill culture, with and without preplant soil fumigation (2 methyl bromide : 1 chloropicrin, 392 kg·ha-1). Plants grown in nonfumigated soil yielded 57% and 46% of the fruit produced by plants on adjacent fumigated soil for cultivars from California and other North American origins, respectively. Plants in nonfumigated soils also developed fruit with lower berry weight (94% and 95% of fumigated trials) and smaller spring plant diameter (83% and 76%) for California and other sources, respectively. Trait values for exotic cultivars ranged from 39% to 80% of those for California cultivars, and the variance component due to germplasm sources explained 41% to 81% of the phenotypic variance of random effects in the experiment. Conversely, significant germplasm source × fumigation interactions were not detected for any of the growth or performance traits evaluated, and the proportion of variance attributable to these interactions was at most 2% of that due to germplasm source. These results demonstrate that strawberry growth and productivity for California and other North American germplasm sources are increased similarly by fumigation. Despite differing selection history, germplasm developed outside of California contains no obvious genetic diversity useful for developing cultivars specifically adapted to the sublethal effects of organisms in nonfumigated soils.

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Kirk D. Larson and Douglas V. Shaw

Bare-rooted `Camarosa' strawberry runner plants were established in a fruit production field on 1 Nov. 1993 using annual hill culture and two preplant soil fumigation treatments: 1) a mixture of 2 methyl bromide: 1 chloropicrin (wt: wt, 392 kg·ha-1) injected into the soil before forming raised planting beds (MBC); or 2) nonfumigation (NF). At about 33-day intervals between mid-January and the end of May, 20 plants were destructively sampled from each treatment to determine leaf dry mass (LDM), crown dry mass (CDM), root dry mass (RDM), and shoot: root dry mass (SRDM) ratios. Plant mortality was <0.2% throughout the study and did not differ with soil treatment. Regardless of sampling date, LDM, CDM, and RDM were greater for MBC plants than for NF plants, although treatment differences were not always significant. During the first 143 days, NF plants allocated a greater proportion of dry matter to roots than to shoots compared to MBC plants, indicating that roots are a stronger sink for photoassimilate in nonfumigated than in fumigated soils. However, there was no difference between treatments in SRDM by the end of the study. Fruit yield and a 10-fruit weight were determined at weekly intervals from mid-January until 23 May 1994. Yield and mean fruit weight of NF plants were 72% and 90%, respectively, of that of MBC plants. For both treatments, about one-half of total fruit production occurred between 144 and 174 days after planting (late March to late April). During that same period, rates of dry matter accumulation in leaf, crown, and root tissues decreased for plants in both treatments, but greatest reductions occurred in NF plants. Chemical name used: trichloronitromethane (chloropicrin).

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Kirk D. Larson and Douglas V. Shaw

Performance traits for twelve strawberry genotypes (Fragaria × ananassa) were evaluated in annual hill culture, with and without preplant soil fumigation (methyl bromide/ Chloropicrin 67:33, 350#/A) at two location. One trial followed several cycles of strawberry plantation whereas the other had not been planted to strawberries for over 20 years. Plant mortality was less than 3%. thus the main effects of fumigation treatment in these experiments must be due to sublethal effects of soil organisms. Plants grown in nonfumigated soil yielded 57% and 51% of the fruit produced by plants on adjacent fumigated soil, for “new” and “old” strawberry ground respectively. Highly significant (P<0.01) differences were also detected for fruit weight (88% and 93%) and leaf number after plantation establishment (73% and 80%). Significant genotype × fumigation interaction was not detected for any of the Performance traits. These results demonstrate that strawberry productivity is substantially increased by fumigation, even in the absence of lethal pathogens. More importantly, little opportunity exists for developing cultivars specifically adapted to nonfumigated soils.

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Kirk D. Larson, Steven T. Koike and Frank G. Zalom

Strawberry plants (`Commander') were grown with different polyethylene bed mulches in the 1999-2000 and 2000-2001 production seasons to determine the effect of mulch on plant growth, yield performance and incidence of Type III strawberry fruit bronzing (T3B), a fruit disorder of unknown origin. In 1999-2000, T3B incidence ranged from 1.8% to 3.7% of total yield, and use of clear, full-bed (CFB) mulch resulted in significantly less T3B incidence than either clear center-strip mulch (CS), or yellow-on-black full-bed mulch. Plant canopy vegetative growth and shoot to root dry mass ratios were greatest for CFB compared to other mulch treatments, but there was no effect of mulch treatment on yield or fruit size. Winter temperatures in 2000-2001 were colder than in 1999-2000, with reduced vegetative growth and increased T3B incidence in spring for all mulch treatments. Use of CFB mulch resulted in greater vegetative growth, greater yield, increased fruit size and reduced T3B incidence compared to CS or green full-bed mulch, but there was no difference among mulch treatments for number of T3B fruit per plot for any single fruit harvest interval. In 2000-2001, the onset of severe T3B symptoms on 7 May was preceded by a brief period of ambient temperatures >31 °C. For all treatments, peak T3B incidence occurred from late May to mid-June, a period characterized by high ambient temperatures and high irradiance conditions. Results indicate that temperature and radiation are significant factors in the development of T3B, and that increased plant vegetative growth in winter results in greater yields and a lower percentage of T3B-affected fruit, particularly in years with cold winters. Managing strawberry plantations to optimize plant growth and development in winter appears to be an effective strategy for reducing the severity of this disorder.

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Kirk D. Larson, Bruce Schaffer and Frederick S. Davies

The effect of flooding on container-grown `Tommy Atkins' mango (Mangifera indica L.) trees on two rootstock, and on container-grown seedling `Peach' mango trees, was investigated by evaluating vegetative growth, net gas exchange, and leaf water potential. In general, flooding simultaneously reduced net CO2 assimilation (A) and stomatal conductance (gs) after 2 to 3 days. However, flooding did not affect leaf water potential, shoot extension growth, or shoot dry weight, but stem radial growth and root dry weight were reduced, resulting in larger shoot: root ratios for flooded trees. Mortality of flooded trees ranged from 0% to 45% and was not related to-rootstock scion combination. Hypertrophied lenticels were observed on trees that survived flooding but not on trees that died. The reductions in gas exchange, vegetative growth, and the variable tree mortality indicate that mango is not highly flood-tolerant but appears to possess certain adaptations to flooded soil conditions.

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Kirk D. Larson, Bruce Schaffer and Frederick S. Davies

One-year-old potted `Peach' mango (Mangifera indica L.) trees were flooded at soil temperatures of 15, 22.5 or 30°C. Hypertrophied lenticels were observed after 5-6 days at 30°C and 6-8 days at 22.5°C, but were not observed after 30 days at 15°C. Cells of hypertrophied lenticels were more spherical and randomly arranged than those of nonhypertrophied lenticels, resulting in increased intercellular airspace. Lenticel hypertrophy also occurred on sterns of trees which were kept moist from intermittant misting, and on excised and intact stem sections. Therefore, formation of hypertrophied lenticels in mango occurs independently of root anaerobiosis and is dependent on floodwater temperature.

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Kirk D. Larson, Douglas V. Shaw and Jerry Sterrett

Three preplant soil fumigation treatments were applied to a strawberry fruit production field in Summer 1993: 1) a mixture of 67 methyl bromide: 33 chloropicrin (wt/wt, 392 kg·ha–1) (MBC); 2) chloropicrin (trichloronitromethane, 336 kg·ha–1) followed by metam sodium (935 liters·ha–1) CMS); and 3) nonfumigation (NF). Bare-rooted `Camarosa' strawberry plants were established in each treatment on 1 Nov. in annual hill culture. Plant mortality was <1%; thus, differences in growth and productivity among treatments were due to sublethal effects of competitive soil organisms. Fruit yields were recorded weekly from 14 Jan. to 23 May 1994. For the NF treatment, early season (January–March), late season (April–May), and total yields were 86%, 69%, and 72%, respectively, of those of the MBC treatment. Early season yields were greatest for the MBC treatment, but late and total yields were greatest for the CMS treatment. From Jan. through May 1994, 20 plants were destructively harvested from each treatment at about monthly intervals for determination of leaf (LDW), crown (CDW), and root dry weight (RDW). For a given date, LDW, CDW, and RDW of plants in the MBC and CMS treatments were greater than those of the NF plants. From January to March, plants in the NF treatment allocated a proportionally greater amount of dry matter to roots, and proportionally less dry matter to crowns and leaves than fumigated plants. In April and May, root: shoot ratios were similar for all three treatments. These data demonstrate the marked influence of soil fumigation treatment on yield and dry matter partitioning of strawberry, and suggest that combinations of chloropicrin and metam sodium may be a viable, albeit expensive, alternative to fumigation with methyl bromide.

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Kirk D. Larson, Bruce Schaffer. and Frederick S. Davies

The influence of floodwater dissolved O2 content on stem lenticel hypertrophy and endogenous ethylene evolution from mango trees, and the influence of exogenous ethylene on mango stem lenticel hypertrophy was examined. In general, floodwater O2 contents of 1-7 ppm resulted in lenticel hypertrophy within about 6 days of flooding, whereas floodwater O2 contents of 15 ppm delayed hypertrophy until about day 9. After 14 days of flooding, there were more than twice the number of hypertrophied lenticels per tree with floodwater O2 contents of 1-7 ppm than with O2 contents of 15 ppm. Ethylene evolution from aerobic stem tissue increased 4- to 8-fold in trees exposed to floodwater with 1-2 ppm O2, increased 2-fold for trees exposed to 6-7 ppm O2, but remained constant with 15 ppm floodwater dissolved O2 content. During a 10-day flooding period, trees in floodwater with 15 ppm dissolved O2 content, and given exogenous ethylene, developed extensive stem lenticel hypertrophy, whereas no hypertrophy developed on stems of trees receiving no exogenous ethylene and maintained in floodwater with 15 ppm O2. These data suggest that ethylene plays a role in promoting stem lenticel hypertrophy in flooded mango trees.

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Thomas R. Gordon, Douglas V. Shaw and Kirk D. Larson

Previous studies have demonstrated significant genetic variation for susceptibility to verticillium wilt, caused by Verticillium dahliae, among strawberry (Fragaria ×ananassa Duch.) genotypes adapted to California growing conditions. These evaluations have been conducted using a conidial root-dip inoculation procedure; valid application of this method in a breeding program assumes the reaction of inoculated plants will be predictive of their response to infection by more natural means. To test this expectation, we evaluated the responses of plants representing eight strawberry genotypes that were either root-dip inoculated prior to being transplanted into a fruit production field or were transplanted into soil artificially infested with pathogen propagules (microsclerotia). Both inoculation methods revealed significant variation among genotypes in all 3 years that tests were conducted and the absence of significant genotype × treatment interactions demonstrate similar rankings of genotypes with both methods. However, based on statistical repeatability, the root-dip inoculation method was more consistent over time (R = 0.759) than the soil inoculation method (R = 0.510).

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Douglas V. Shaw, Thomas R. Gordon and Kirk D. Larson

Strawberry runner plants from the cultivar `Selva' (Fragaria ×ananassa Duch.) were produced using three nursery treatments in each of three years: propagation in soil fumigated with a mixture of 2 methyl bromide: 1 chloropicrin (w/w) at 392 kg·ha-1, propagation in fumigated soil but using planting stock inoculated prior to nursery establishment with a conidial suspension of Verticillium dahliae (106 conidia/mL), and propagation in nonfumigated soil naturally infested with V. dahliae. Runner plants were harvested and stored at 1 °C for 6, 18, or 34 days prior to establishment in fruit production trials. No significant differences were found between runner plants grown in naturally infested soil and runner plants obtained from artificially inoculated mother plants for V. dahliae infection rates detected by petiole isolation immediately prior to transplanting, the percentage of plants visibly stunted due to disease during the following production season, and seasonal yield compared with corresponding noninfected controls. Cold storage of runner plants for 18 or 34 days, produced using either natural or artificial inoculation systems, reduced the initial percentage of infected plants by 42% to 61% and the percentage of stunted plants during the following fruit production season by 43% to 57%, compared with plants from corresponding nursery treatments given only 6 days post-nursery cold storage. Yields for inoculated plants with 6 days cold storage were 16% to 20% less than those for uninoculated controls, whereas yields for inoculated plants with 18 or 34 days of storage were 3% to 9% less than the respective controls. Most of the cold storage effects on initial infection rate, stunting, and yield were realized at the 18 days of storage treatment. A reduction in the fraction of V. dahliae infected plants due to cold storage, suggests either a direct effect of cold storage on the disease organism or stimulation of secondary resistance mechanisms in the plant. Chemical name used: trichloronitromethane (chloropicrin).