Runner Plants Research Articles

First Report of Colletotrichum acutatum on Strawberry in Finland.

Strawberry (Fragaria × ananassa) is the most important small fruit crop in Finland. The quarantined pest Colletotrichum acutatum was detected for the first time on strawberry in August 2000, in Eastern Finland. Waiting-bed plants of cultivar Elsanta had symptoms typical of anthracnose rot (black spot) on the fruit, and small black lesions were seen on stolons. The rainy, warm, and humid weather of the summer favored the development and spread of the disease. C. acutatum was isolated from lesions on the fruit. Abundant sporulation was observed on lesions, and the fungus was readily isolated on potato dextrose agar (PDA) from surface-sterilized fruit pieces. Spores (11.5 to 19.8 × 3.3 to 4.0 μm) were acute at both ends, and acervuli with salmon pink spore masses matched the descriptions of C. acutatum (1). At first, the colonies were white and later became ash gray. The growth rate of the fungus was 8.0 to 8.8 mm per day, at 27°C. The identification was confirmed with enzyme-linked immunosorbent assay at the Central Science Laboratory, York, U.K. In order to fulfill Koch's postulates, young potted strawberry plants were inoculated by misting with a suspension of 10.4 × 106 conidia per ml of C. acutatum. Healthy runner plants of micropropagated cultivars Bounty, Cudaruska, Jonsok, Korona, and Oka (10 plants of each cultivar) were inoculated and incubated for 3 days, at 100% relative humidity. After 3 weeks at 20 to 21°C and 16 h of light per day, dark, elongated lesions measuring 2 to 15 mm were observed on the stolons. All tested cultivars developed symptoms on stolons and petioles, but no crown infections were detected. Flowering and fruit-bearing plants of cultivars Honeoye, Sara, and Senga Sengana were similarly inoculated and incubated. The first symptoms of fruit rot were detected 5 days after inoculation on the ripening fruit of Honeoye. Less severe fruit rot was seen on the fruit of Senga Sengana and the Fragaria vescana cultivar Sara. Salmon pink sporodochia developed on infected fruit, stolons, and leaves, within 5 days on moist filter paper, and C. acutatum was isolated on PDA. No other pathogens were present. Because C. acutatum is a quarantined pest in Finland, all strawberry plants of the infected lot on this particular farm were destroyed and future use of the field will be restricted. Reference: (1) C. M. Howard et al. 1992. Plant Dis. 76:976-981.

Winter hardiness of micropropagated and conventionally propagated strawberry plants

SummaryEnhanced vigour of micropropagated strawberry (Fragaria X ananassa Duch.) plants has given cause to suspect their winter hardiness to be inferior to that of conventionally propagated plants. This experiment was carried out to examine and compare cold hardiness and overwintering of three types of strawberry plants of cultivars Senga Sengana and Jonsok: 1) micropropagated virus-free elite plants, 2) certified plants (runner plants from elite plants), and 3) ordinary plants (runner plants of conventionally propagated plants from a strawberry farm). A special “double pot”-method was developed and used in planting so that the plants could be lifted for controlled freezing tests during winter. Additionally, winter survival was evaluated in the field during flowering. In controlled freezing tests, strawberry crowns survived -8°C without injury, suffered minor injury at -10°C and severe injury at -12°C. No consistent differences in cold hardiness among the three types of plants were detected during winter. Field evaluation did not reveal any differences in their winter survival, either. Micropropagated plants flowered more freely than did the plants produced through runners.

Soil Fumigation and Runner Plant Production: A Synthesis of Four Years of Strawberry Nursery Field Trials

Strawberry (Fragaria ×ananassa L.) runner plant production during a 4-year period was compared on nursery soils treated with methyl bromide (MB) and chloropicrin (CP) mixtures (MB:CP) and three alternative soil treatments: CP, mixtures of 1,3-dichloropropene (Telone®) and CP (DP:CP), and no fumigation (NF). The effect of soil treatment on runner plant production for a single nursery propagation cycle was determined in all 4 years. In 2 years, runner production in a final propagation cycle was also determined as a function of soil treatment in previous cycles. A single propagation cycle in NF soil decreased runner production relative to all other treatments. Treatments with CP at rates of 140 to 191 kg·ha–1 generally decreased runner production significantly (P ≤ 0.05) in comparison with treatment with MB:CP; use of CP at rates ≥303 kg·ha–1 resulted in statistically equivalent runner production. In one trial, use of two DP:CP formulations (516 kg·ha–1 of a 7:3 DP:CP mixture, and 448 kg·ha–1 of a 3:7 DP:CP mixture) significantly reduced and did not affect runner production, respectively, relative to the use of MB:CP. Use of MB:CP in the previous propagation cycle also increased runner productivity in comparison with NF. Runner productivity of planting stock produced with 314 kg·ha–1 of CP did not differ statistically from that of stock produced with MB:CP, but productivity of planting stock on soil treated with 157 kg·ha–1 of CP was intermediate between that on NF and MB:CP-treated soil. Planting stock grown on nontreated soil in two previous propagation cycles produced 25% fewer runner plants than did similar stock grown on MB:CP-treated soil. Productivity of planting stock produced with CP at rates of 280 to 314 kg·ha–1 in two previous propagation cycles did not differ statistically from that of stock produced with MB:CP. Results of meta-analyses indicated that fumigation with MB:CP was more effective in increasing runner production than was CP or NF, regardless of the propagation cycle or rate of application. For mixtures of 1,3-dichloropropene and CP, nursery productivity was maximized by using at least 280 kg·ha–1 of CP.

Matted-row Strawberry Production using Landscape Fabrics and Organic Mulch for Weed Control

Three landscape fabrics, Magic Mat®, a heavy black plastic woven fabric with a fuzzy underside; Weed Mat®, a thin black plastic sheet with small holes; and Typar®, a dark gray spun bonded material, with and without a cover of organic oak bark mulch, were evaluated for weed control and ability of strawberry plant roots to establish through the fabrics over a 4-year period. Landscape fabrics reduced weed numbers for the first 3 years in comparison with the bare ground treatment. With few exceptions. the organic mulch did not improve the weed control capability of landscape fabrics. Fruit yield for the Weed Mat and Magic Mat treatments did not differ from the bare ground treatment, but was lower for the Typar treatment when averaged over organic mulch treatments. Fruit yield was higher where the organic mulch was used when averaged over all landscape fabric treatments. Fruit size was slightly larger for the bare ground and smallest for the Typar treatments during the first harvest season, but there was no difference in fruit size by the third year of harvest. Fruit size for the organic mulched plots was slightly larger than that for the unmulched plots the second year of harvest, but there was no difference for the first or third years. The number of strawberry runner plants that rooted and plant row vigor were greater for the Weed Mat, Magic Mat and plots without the landscape fabric than for the Typar plots, particularly in the second and third season. Rooting of runner plants and plant row vigor was better with organic mulch. Landscape fabric tended to reduce extent of rooting, especially in the first season, but it was improved by the application of organic mulch.

577 Strawberry Runner Plant Root Pruning and Subsequent Growth and Yield in California

Each year, 500,000,000 bare-root plants (crowns) are used to establish strawberry plantings in California. Careful placement of plants in the planting holes is a critical determinant of plant growth and productivity, but large-rooted crowns are difficult to plant correctly. Use of uniform plant material would facilitate proper planting. I conducted a study in 1997–98 to determine the influence of three root pruning treatments on performance of `Camarosa' bare-root strawberry plants. Crowns were machine-dug from a nursery on 27 Oct. 1997. Two-hundred-forty plants were selected for uniformity of size, and root systems of all selected plants were >12.7 cm in length. Root systems of individual plants were randomly subjected to three pruning treatments: nonpruned (NP), pruned to 8.9 cm in length (P1), or pruned to 4.45 cm in length (P2). Twenty plants were randomly selected from each pruning treatment, washed, divided into excised and intact root tissues, and dried at 70 °C for 6 days. Pruning resulted in the removal of 23% and 78% of root dry mass for plants in the P1 and P2 treatments, respectively. For each treatment, the 60 remaining plants were established in raised beds at the Univ. of California South Coast R.E.C. in Irvine on 1 Nov. 1997. Experimental design was a RCB, with one 20-plant plot per treatment in each of three replicate blocks. There was no plant mortality and no difference among treatments in plant canopy diameter in March (mean diam. = 36.7 cm). Fruit yields were determined for each plot at weekly intervals from 1 Feb. to 18 Apr. 1998. There was no effect of pruning on yield or fruit size, suggesting that strawberry root systems have considerable regenerative ability.

Effect of number of plants per plant hole and of runner plant crown diameter on strawberry yield and fruit mass

Runner plants of three strawberry cultivars, ‘Selekta’, ‘Tiobelle’ and ‘Tioga’, each with three crown diameter classes, were planted using one, two or three runner plants per planting hole. This was done in order to determine whether yield reductions stemming from the use of small runner plants (crown diameter <5 mm) could be prevented by planting more than one small runner plant per hole. Overall yields increased as the number of plants per hole increased for all three crown diameter classes. Crown diameter> 10 mm and crown diameter 5–10 mm runners of ‘Selekta’ and ‘Tiobelle’ gave similar yields. ‘Tioga’ crown diameter <10 mm runner plants yielded better than crown diameter <5 mm plants. Increasing the number of plants per hole resulted in a mean fruit size decrease of 8.9. These results indicate that the use of more than one runner plant of small crown diameter per planting hole may result in improved yields, but not economic returns with existing price structure of runner plants. Nurseries should, therefore, consider selling small runner plants at reduced prices in order to encourage this potentially mutually beneficial practice.

Effects of Light Intervals on Flower-Bud Formation, Leaf Growth, and Chlorophyll and Carbohydrate Concentrations in "Nyoho" Strawberry Runner Plants during Storage under Cool Conditions.

Runner plants of “Nyoho”strawberry plants were exposed to different light intervals during storage ; i. e., day/night cycles of 8/16 h, 8/16 h for 3 d followed by 0/24 h, 8/40 h, 8/88 h, and continuous darkness, and the effects on flower-bud formation, leaf growth, and chlorophyll and carbohydrate concentrations were investigated. Succulent elongation of petioles, decreased leaf area and etiolation of leaflets improved as light intervals were shortened. Although carbohydrate concentrations and percentage of flower-bud formation in continuous darkness were lower than in plants with 8/16 h, these values did not decrease even when the dark intervals became prolonged to 8/40 h.

Response of Injury to Strawberry Crowns Caused by European Corn Borer Larvae

Larvae of several insects injure and kill strawberry (Fragaria ×ananassa Duchesne) plants by burrowing into and hollowing out plant crowns. Occasionally, these infestations are serious enough to cause heavy economic losses to fruit producers and nursery plant growers. In 1997 in Beltsville, Md., we observed wilting and dying mature plants and unrooted runner plants in two experimental strawberry plantings. Injury by larvae was extensive; large cavities occurred in crowns, and the central pith tissues were removed from stolons and leaf petioles. Often, insect frass was seen at entrance holes. Larvae removed from hollowed-out parts of injured plants were identified as the European corn borer (Ostrinia nubilalis Hübner) in their fifth instar stage. Their presence in this instance also was associated with a cover crop of millet [Setaria italica (L.) P. Beauv., `German Strain R'] planted between the strawberry rows for weed suppression. This is the first published report of the European corn borer attacking strawberry. Although this insect may occur only sporadically in strawberry plantings, it may become important in the future. Growers and other professionals should become aware of this new strawberry pest and recognize that its management in strawberry will be different from management of other crown-boring insects.

Injury to Strawberry Crowns Caused by European Corn Borer Larvae

Larvae of several insects injure and kill strawberry (Fragaria ×ananassa Duchesne) plants by burrowing into and hollowing out plant crowns. Occasionally, these infestations are serious enough to cause heavy economic losses to fruit producers and nursery plant growers. In 1997 in Beltsville, Md., we observed wilting and dying mature plants and unrooted runner plants in two experimental strawberry plantings. Injury by larvae was extensive; large cavities occurred in crowns, and the central pith tissues were removed from stolons and leaf petioles. Often, insect frass was seen at entrance holes. Larvae removed from hollowed-out parts of injured plants were identified as the European corn borer (Ostrinia nubilalis Hübner) in their fifth instar stage. Their presence in this instance also was associated with a cover crop of millet [Setaria italica (L.) P. Beauv., `German Strain R'] planted between the strawberry rows for weed suppression. This is the first published report of the European corn borer attacking strawberry. Although this insect may occur only sporadically in strawberry plantings, it may become important in the future. Growers and other professionals should become aware of this new strawberry pest and recognize that its management in strawberry will be different from management of other crown-boring insects.

Agrobacterium-mediated transformation of `Alpine' Fragaria vesca, and transmission of transgenes to R1 progeny.

Agrobacterium-mediated transformation was used to stably introduce β-glucuronidase (gus) and neomycin phosphotransferase (nptII) marker genes into `Alpine' Fragaria vesca FRA 197, a diploid (2n = 2x = 14) strawberry. R0 generation transformants derived from a single clump of kanamycin-resistant callus were vegetatively propagated. The presence of the gus and nptII genes in five clonal R0 runner plants was confirmed by PCR. Southern analysis suggested two sites of nptII insertion. When R1 generation seedlings obtained via self-fertilization of R0 plants were tested by histochemical assay, 591 were GUS positive and 39 were GUS negative. The R1 segregation data fit a 15 : 1 ratio (0.5 > P > 0.25), consistent with the independent segregation of two transgene insertion loci. These results demonstrate the suitability of `Alpine' F. vesca for transgene research in strawberry.

休眠期における四季成り性イチゴの栄養生長と生殖生長を誘導する温度と日長条件

Effects of temperature and photoperiod on vegetative and reproductive growth were investigated in everbearing strawberry (Fragaria×ananassa Duch. cv. 'Summerberry') plants. Runner plants grown in a nursery field until November were transplanted to clay pots, and were grown in growth chambers at a 12-hr day/night setting of 20/15°, 25/20°, and 30/25°C, under either constant light (24-hr, long-day) or an 8-hr (short-day) photoperiod. The treatments were started on December 1. 1. At 30/25°C, the appearance of an inflorescence was inhibited under short-day. By contrast the number of inflorescences was remarkably increased until 10 weeks after the start of the long-day treatment. The production rate of axillary inflorescences which developed from subtended axillary bud without a foliage leaf was increased remarkably under long-day. These results were similar to those of non-dormant plants. 2. At 20/15°C and 25/20°C, more inflorescences were initiated under the long-day than under the short-day treatment. At 20/15°C, the photoperiod was not effective on the development of inflorescences. 3. Under the 8-hr photoperiod, the petioles grew longer at 25/20° and 30/25° than those on plants kept at 20/15°C ; conversely, under the 24-hr photoperiod, the average length of the petioles became shorter at the higher temperatures with time compared to that of plants kept at 20/15°C.

Response to Family Selection for Field Resistance to Verticillium dahliae in California Strawberries

Strawberry genotypes were retained from biparental progenies previously identified as either relatively susceptible or resistant to wilt caused by Verticillium dahliae based on a phenotypic resistance score. Runner plants from these selected genotypes were inoculated with a conidial suspension containing a mixture of five isolates obtained from symptomatic strawberry plants at 106 conidia/mL, then scored for disease symptoms. Genotypes from resistant progenies had significantly higher phenotypic resistance scores (1 = severe symptoms, 5 = no detectable symptoms) than those from susceptible progenies (4.15 vs. 2.23), and there also was a correlated selection response for the percent plants severely stunted or dead (26.4 and 69.1 for resistant and susceptible selections, respectively). Comparisons of the resistance scores for selected groups with those for the original parents (2.76) indicated that selection had changed relative resistance significantly in both directions and that realized response had been 24% to 43% larger than predicted for selection in both directions. Although several of the selections from resistant progenies were highly susceptible, five of the 21 resistant selections had resistance scores outside the range of the original parents, representing possible transgressive segregants. This comparison is limited by the precision with which individual resistance scores are estimated and by the scope of the disease symptoms in this trial. Detecting genotypes with sufficient resistance ultimately will depend on development of screening methods with greater sensitivity than those used here.

Field Resistance of California Strawberries to Verticillium dahliae at Three Conidial Inoculum Concentrations

Runner plants from strawberry (Fragaria ×ananassa Duch.) genotypes previously identified as relatively susceptible, intermediate, and resistant to wilt caused by Verticillium dahliae Kreb. were inoculated with a conidial suspension containing a mixture of five isolates of V. dahliae at 104, 105, and 106 conidia/mL. Disease symptoms were scored as the number of dead or severely stunted plants per plot, or on the basis of a subjective phenotypic resistance score assigned to each plot on eight dates during the spring after planting. Overall disease symptoms increased with inoculum concentration; resistance scores for all genotypes were 4.8, 3.7, and 3.2, and the percentages of plants stunted or dead were 6.8, 32.6, and 44.9 for the three conidial concentrations, respectively. The relative resistance categories were separable at concentrations of 106 and 105, whereas no separation was obtained at 104 conidia/mL. Genotypes originally classified as intermediate in resistance performed more like susceptible types at the highest conidial concentration. Significant resistance category × conidial concentration level interactions were detected for resistance score but not for the number of dead or severely stunted plants per plot. Regression coefficients for resistance score and percentage of stunting on conidial concentrations were statistically significant only for susceptible and intermediate genotypes. Some stunting was detected within all resistance categories at the highest conidial concentration, and the practical value of the resistance developed to date will depend ultimately on realistic field inoculum levels.

Nursery Selection for High Early Fruit Yield in Subtropical Strawberries

Development of new strawberry cultivars for winter fruit production in Florida entails growing of hybrid seedlings in a nursery in the summer of the first year for runner plant production. Two runner plants are taken from each seedling and planted in the fruiting field in the fall. An experiment was conducted to see if it is possible to predict which genotypes in the nursery will have the highest early season fruit yield. Seedlings from 24 families from a 6 × 4 factorial mating design were grown in a nursery. From each family, daughter plants of 20 seedlings with the highest vigor and 20 randomly picked seedlings were then evaluated in the fruiting field. Plants from selected (high-vigor) seedlings were more vigorous, but had fewer crowns and runners, than unselected plants. More inflorescences were counted in selected plants than in unselected plants during the second week of January. This could account for higher early yield (yield at the end of January) and total yield (yield at the end of March) in selected than in unselected plants.

Polyethylene Mulch, Diurnal and Seasonal Soil Temperatures, and Growth and Productivity of Strawberries in Southern California

Southern California strawberry growers use clear polyethylene mulch to increase soil warming and promote plant growth and fruiting, but use of clear poly mulch is only feasible when effective preplant soil fumigation controls weeds. In the absence of methyl bromide fumigation, the use of wave-length selective (WLS) or black polyethylene bed mulches may be required for adequate weed suppression, but the influence of these materials on strawberry plant growth and productivity in southern California is not well-documented. We conducted experiments in 1994–95 and 1995–96 to determine the influence of various mulch formulations on soil temperature and growth and productivity of `Chandler' strawberry in Irvine, Calif. Clear poly and a green WLS material (IRT76, AEP Plastics) were compared in both trials; in addition, the 1995–96 trial included a brown WLS material (ALOR, PolyWest, Inc.) and a black poly mulch. For both trials, freshly dug runner plants were established in premulched beds in early October, and soil temperatures were continuously monitored at a 10-cm depth using thermocouples and a recording datalogger. Fruit harvest commenced in December and continued through June. In both years, clear poly mulch resulted in significantly greater soil temperatures, greater December plant diameters, and greater early and total fruit yields than other mulches. In both years, use of clear poly resulted in 12% greater fruit yields than the other three materials. No growth or productivity differences were observed among the WLS and black mulches, although differences were observed in mean soil temperatures.

Pectic enzymes as a selective pressure tool forin vitro recovery of strawberry plants with fungal disease resistance

A novel strategy in selecting strawberry (Fragaria xananassa L.) plants with resistance toRhizoctonia fragariae andBotrytis cinerea was developed. Purified pectic enzymes produced byR. fragariae were usedin vitro to select morphogenetic calluses. Both regenerated shoots and plants were testedin vitro andin vivo withR. fragariae andB. cinerea. Thein vitro resistance of shoots regenerated under selection pressure was confirmed byin vivo tests with runner plants either by root immersion in a suspension ofR. fragariae mycelium before potting the plants in sterile soil, or by spraying the leaves with several strains ofB. cinerea spores. The increase of resistance against pathogens was correlated to the increase of phenolic compounds, particularly orthodibydroxyphenols.

The Effect of Gibberellic Acid on the Production and Quality of Strawberry Runners

In this study, the effect of GA3 on the production and quality of strawberry runners per area were examined. Cold stored of the planting stocks of Cruz, Vista, Tufts, Pocahontas and Aliso varieties were used as plant materials. In order to determine the optimum application time and rate, GA3 was applied to spring planted mother plants 30 and 60 days after planting at the concentration of 50 and 100 ppm. The number of runners per m2 and stem diameter were increased significantly with GA3 application, whereas GA3 did not affect the length of root. Cruz which has low runner production characteristic, became one of the best variety on runner production with GA3 application. Pocahontas gave the best response to GA3 application. Varieties which has the ability to produce high number of runners, also yielded with higher number of runner plants than the contral plants.

Soil Fumigation, Fruit Production, and Dry Matter Partitioning of Field-grown Strawberry Plants

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 &lt;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).

Strawberry Growth and Weed Control in Response to Using Corn Gluten Hydrolysate

Corn gluten meal (CGM), a by-product of corn wet-milling, has weed control properties and is a N source. The weed control properties of CGM have been identified in previous studies. The hydrolysate is a water soluble, concentrated extract of CGM that contains between 10% to 14% N. Our objective was to investigate corn gluten hydrolysate as a weed control product and N source in `Jewel' strawberry production. The field experiment was a randomized complete block with a factorial arrangement of treatments with four replications. Treatments included application of granular CGM, CGM hydrolysate, urea, urea and DCPA (Dacthal), and a control (no application). Granular CGM and urea were incorporated into the soil at a depth of 2.5 cm with N at 0, 29, 59, and 88 g/plot. Plot size was 1 × 3 m. Percent weed cover data on 12 Aug. showed plots receiving the 29 g N from CGM hydrolysate had 48% less weed cover than the control (0 g). Plant growth variables showed similar numbers of runners and runner plants among all nitrogen sources.

Genetic Variation for Field Resistance to Verticillium dahliae Evaluated Using Genotypes and Segregating Progenies of California Strawberries

Resistance to wilt caused by Verticillium dahliae Kreb. was evaluated for 41 strawberry genotypes from the Univ. of California breeding program and 1000 offspring from crosses among 23 of these genotypes. Runner plants from these genotypes and seedlings were inoculated with a conidial suspension containing a mixture of five isolates of V. dahliae from strawberry. Symptoms were scored as the number of dead or seriously stunted plants per plot, or based on a subjective phenotypic resistance score assigned to each plot on five dates during the spring after planting. Most of the California germplasm is highly susceptible to V. dahliae, with an average resistance score of 2.1 (±0.10) and 84.1% (±2.1) plants stunted or dead compared with a score of 3.2 (±0.24) and 57.4% (±4.9) of plants stunted or dead for a control set of six non-California genotypes identified previously as resistant. However, a broad range of intermediate resistance was detected, and 4 of the 41 California genotypes evaluated had resistance scores superior to the mean score for the non-California resistant checks. Plot-mean heritabilities for resistance and stunting scores estimated using genotypic, full-sib family, and offspring-parent analyses ranged from 0.44 to 0.88. Comparison of different estimates of variance components suggests that half or more of the genotypic variance for resistance traits detected is due to the additive effects of genes. There appears to be sufficient variation within the California population to proceed with an effective selection program, despite the absence of directional selection for resistance during the past 3 decades. However, developing cultivars with adequate resistance will ultimately depend on the recovery of transgressive segregants from superior parents, as even the most resistant genotypes from all sources showed some disease symptoms.