Non-fumigated Plots Research Articles

Impact of phosphorus rate, soil fumigation, and cultivar on potato yield, quality, phosphorus use efficiency and economic returns in a high phosphorus soil

Phosphorus (P) is a non-renewable resource that will become an economic limiting factor for food production in the future. Potato (Solanum tuberosum L.) is a relatively shallow rooted crop and often responds positively to P fertilizer even in soils with high soil-test P. Therefore, high-rate P fertilizers are commonly applied to produce high tuber yields in commercial potato fields. The purpose of this study was to evaluate the effects of P rate, cultivar, and soil fumigation on potato yield, quality, P uptake, P use efficiency and economic returns under high soil test P conditions. A field study with a split-plot randomized complete block design was conducted in 2021 and 2022, with fumigation treatments (no fumigant or fall-applied metam sodium) being the main plots and cultivars (Ivory Russet or Russet Burbank) as subplots. Each subplot was further divided into five sub-subplots with different P rates: 0, 37, 73, 147 and 220 kg P ha-1. The results indicated that Russet Burbank produced higher total yield than Ivory Russet. Fumigation significantly increased tuber yield and dry matter percentage. Both cultivars responded to P application and tuber P uptake increased with P rate even under very high soil test P conditions. Both partial factor productivity (PFP) and P recovery efficiency (PRE) were very low and significantly decreased with the P rate. Russet Burbank had higher PFP and PRE than Ivory Russet, and fumigated plots had significantly higher PRE than non-fumigated plots. The tuber yield output and net revenue of Russet Burbank was higher than Ivory Russet, and the highest economic return was obtained in fumigated plots at 147 kg P ha-1. These results suggest that cultivar selection and control of soil-borne diseases should be considered to optimize P input, tuber yield, and to conserve P resources.

Root-knot nematode damage to a cucurbit double crop is increased by chloropicrin fumigation on the previous tomato crop.

Double-cropping is a common practice in vegetable plasticulture whereby a second crop is planted on the same plastic bed as the first crop. Root-knot nematodes (Meloidogyne spp.) are one of the major soilborne constraints in double-cropped vegetables due to nematode population build-up on the first crop. We evaluated the effect of fumigant and non-fumigant nematicides applied on the first crop, on nematode infection and yield of the second crop in 10 field trials between 2017 and 2020. Fumigants were chloropicrin (Pic100), chloropicrin +1,3-D (PicClor60), and non-fumigant nematicides were oxamyl (Vydate), fluensulfone (Nimitz), fluopyram (Velum) and fluazaindolizine (Salibro). The first crop was tomato and double crops were cucumber, squash, zucchini, and cantaloupe. Fumigation with chloropicrin on the first crop increased root-knot nematode damage on the double-crop at the end of the season in seven trials, while the opposite was noted in one trial, and no difference was noted in two trials. Fumigation with chloropicrin+1,3-D resulted in root-knot nematode damage less than chloropicrin but more than non-fumigated plots. Cucurbit yield was greater in non-fumigated beds in four trials, and in chloropicrin-treated beds in two trials. Fluensulfone reduced root-knot nematode damage on the second crop in five out of 10 trials. Our results indicate that chloropicrin applied on the tomato crop may lead to increased root-knot nematode damage on the double crop. More research is needed to understand the processes behind this, but it is possibly related to a reduction in natural nematode soil suppressiveness due to the broad-spectrum fungicidal activity of chloropicrin. © 2022 Society of Chemical Industry.

Soil microbial community characteristics in a vineyard ten years after fumigation

AbstractCalifornia is the number one producer of grapes in the United States. Soil fumigation has been used to control soilborne pests and pathogens. However, fumigants can potentially have a negative impact on native soil microbial populations. Given the roles of microorganisms in the soil system, it is important to assess the long‐term impact of fumigation on soil microbial communities, which is currently lacking. The objective of this research was to study the long‐term impact of microbial communities in grapevine (Vitis vinifera L.) replanting situations after fumigation. Soil treatments included methyl bromide (MeBr), Telone C35, InLine, a nonfumigated control, and a mustard (Sinapis alba L.) cover crop in two side‐by‐side field experiments fumigated in 2007 and 2008. Microbial community biomass in 2017 and 2018 did not differ between fumigated and nonfumigated plots at the 0‐ to 30‐cm depth. However, plots with shank‐injected MeBr and Telone C35 applied in 2008 showed significantly reduced microbial biomass in 2018 at deeper depths than subsurface drip‐applied InLine plots. The results show the impact of fumigation on bacterial communities; however, the impact on fungal and arbuscular mycorrhizal fungi (AMF) communities was not evident after 10 years. Furthermore, the effect of depth was apparent on bacterial communities compared with fungal and AMF communities. The data suggest that deep‐rooted grapevines influenced fungal populations, which probably supplied labile C in fumigated and nonfumigated plots. Future research needs to focus collectively on the long‐term effects of fumigation on fruit quality and yield in vineyards, and on soil microbial communities.

Integrating grafting and emerging products to manage soilborne diseases of tomato

Major soilborne diseases in North Carolina and surrounding states include fusarium wilt (FW) (causal agent Fusarium oxysporum f. sp. lycopersici; all three races), verticillium wilt (VW) (Verticillium dahliae; two races), southern stem blight (SSB) (Sclerotium rolfsii), root-knot nematodes (RKN) (primarily Meloidogyne incognita) and bacterial wilt (BW) (Ralstonia solanacearum race 1), distributed across the subtropical to temperate ecosystems in the state. FW, VW, SSB, and RKN can be well managed using standard fumigants. Alternative management practices are needed in production systems where fumigants are not used or effective, and/or where heirloom tomato cultivars are grown. We are involved in a USA multi-state program to determine the viability of grafting in open-field production systems. Previously published work showed the utility of grafting to manage FW, SSB, RKN, and BW. In complementary work to manage BW, a replicated on-farm field trial demonstrates that fumigants such as Paladin (79% dimethyl disulfide + 21% chloropicrin) and PicClor60 (60% chloropicrin + 40% 1,3-dichloropropene) allow up to 80% plant death, similar to incidence in non-fumigated plots, whereas grafting to resistant rootstock (Seminis 'Cheong Gang') confers 100% control in non-fumigated plots. Another on-farm randomized complete block design experiment demonstrated that three commercially available rootstocks conferred 100% plant stand, whereas non-grafted plants had 80% plant death using a round tomato scion ('Red Mountain') or 15% plant death using a roma tomato ('Picus'). Grafted plants show economic viability in North Carolina, and additional work is needed to optimize this tool in diverse production systems.

Effectiveness of Nontarped Broadcast Fumigation and Root Removal on Root Lesion Nematode and Fusarium and Pythium Species in a Red Raspberry System

Improved methods for the management of soil-borne pathogens are needed in the raspberry production system in the Pacific Northwest. A multiyear experiment was conducted in a commercial raspberry field in northwest Washington with the following objectives: (i) to determine if the standard industry fumigation treatment of nontarped broadcast application of 1,3-dichloropropene/chloropicrin (Telone C-35) is effective against Pratylenchus penetrans and Pythium and Fusarium species; (ii) to determine if root removal prior to nontarped broadcast fumigation with Telone C-35 improves management of P. penetrans and Fusarium and Pythium species; and (iii) to identify microbes that have the potential to be soil-borne pathogens through microbial community analysis. Fumigation alone briefly reduced population densities of P. penetrans but was ineffective at reducing existing, pretreatment populations of Fusarium and Pythium. However, populations of these two genera were lower in fumigated than in nonfumigated plots for at least 6 months after fumigation. Root removal did not enhance fumigant efficacy under the conditions of the experiment. Ilyonectria (Cylindrocarpon) was found to be more dominant in the fungal community of nonfumigated roots and rhizosphere soil compared with fumigated. These results demonstrate that P. penetrans, Fusarium, and Pythium rapidly rebound after nontarped broadcast fumigation in raspberry with the industry standard Telone C-35.

Interactive Effects of Fumigation and Fungicides on Potato Response to Nitrogen Rate or Timing

The availability and use of crop protectants including azoxystrobin in combination with fumigation has extended the Central Wisconsin effective growing season by 2 to 4 weeks. This study, evaluating the influence of these crop protection practices on the optimum nitrogen rate and time of application for Russet Burbank potato (Solanum tuberosum L.), was established as two 3-year field trials designed as split-split plot experiments. Both experiments used metam sodium as the main plots and fungicide treatment (chlorothalonil Zn alone or chlorothalonil Zn alternated with azoxystrobin for the first six sprays) as the first split. In-season fertilizer N rate (179, 224, 269, or 313 kg N ha−1) or in-season N timing (N split into two, three, or four applications at 269 kg N ha−1) was the second split. Not fumigating resulted in significantly higher verticillium ratings and severely repressed crop yield and tuber quality responses to both fungicide treatment and N rate. On average, fumigation increased total yield 13.6 Mg ha−1yr−1, U.S. No. 1 tubers by 9 % and U.S. No. 1 tubers >170 g by 5 % over where fumigation was not used. In 2 of the 3 years when azoxystrobin was included in the fungicide program early blight severity was reduced by about 50 %, and on fumigated areas yields were increased 4.8 Mg ha−1, whereas no yield increase was seen from this fungicide treatment on the non-fumigated plots. In these same years, fumigation increased optimum N rate by about 50 kg ha−1; however, there was no apparent interaction with fungicide treatment. Although fumigation, fungicide treatment, and time of N application each influenced tuber yield or tuber quality in some years, in the two more responsive years some interactions between these factors were statistically significant, with benefits generally only seen where plots were fumigated.

Effects of long-term organic amendments and soil sanitation on weed and nematode populations in pepper and watermelon crops in Florida

Florida vegetable growers have relied on methyl bromide (MeBr) fumigation to manage soil pathogens, nematodes, and weeds. This system combined with raised beds, polyethylene mulch, and seepage and/or drip irrigation has been effective for producing high vegetable yields. Alternatives to MeBr such as solarization and organic amendments have given favorable results in small trials, but there are few large-scale studies. The objectives of this study were to evaluate the effects of long-term organic amendment applications and soil sanitation treatments on weed and nematode populations on pepper (Capsicum annuum L.) and watermelon (Citrullus lanatus [Thunb.] Manst.). During 1998 and 1999 fall vegetable seasons, main plots received a yearly organic amendment (biosolids) application or a non-amendment control, with sub-plots consisting of soil sanitation treatments with solarization, MeBr, Telone® (1,3-dichloropropene), or a non-fumigated control. Each sub-plot was further divided into two sub-sub-plots, one receiving additional weed and without control weed control. During the solarization period (60 d in 1998–1999; 90 d in 1999–2000), percent weed cover was higher in the non-biosolid plots than the biosolid plots for the first part of the solarization period, but there were no differences during the last 30 days in both seasons. Purple nutsedge was able to germinate on the north edge of the beds for a border effect; a point of vulnerability when beds run east–west. With the pepper crop, the number of weeds and percent weed cover were greater in the non-fumigated plots and Telone®-treated plots than in plots treated with MeBr or in solarized plots with and without biosolids. Nematode population densities from plot to plot within the site were highly variable, which likely accounted for the relatively few consistent effects from treatments observed during the experiment. The data do provide some indication of the importance of weeds in the recovery and buildup of nematode populations. During spring 1999, both root-knot and stubby-root nematodes were more abundant in the sub-sub-plots that had not received weed control. The results suggest that solarization and organic amendments can be viable alternatives to MeBr. However, MeBr produced the most consistent results.

Soil Suppressiveness Against the Disease Complex of the Soybean Cyst Nematode and Sudden Death Syndrome of Soybean

The ecology of the complex of soybean cyst nematode (SCN) and sudden death syndrome (SDS) of soybean was investigated under soybean monoculture in two field experiments from 2003 to 2007. Initially, susceptible soybean 'Spencer' was planted while inoculating Fusarium virguliforme into nonfumigated or preseason-fumigated plots (methyl bromide, MB, at 450 kg/ha), and SCN and SDS were monitored. In one field, SCN population densities declined in nonfumigated but increased in fumigated plots. After years of limited SDS in 2003 and 2004, SDS developed later in nonfumigated than fumigated plots. In 2006 in the greenhouse, nondisturbed or disturbed soil cores (10-cm diameter, 30-cm depth) from field plots received two two-level factors: (i) nonfumigated or fumigated (1,070 kg/ha MB); and (ii) noninoculated or inoculated with 9,000 second-stage juveniles of SCN. At harvest, nonfumigated cores from nonfumigated plots had fewer nematodes and less SDS regardless of disturbance or inoculation than the corresponding fumigated cores and any cores from fumigated plots. In the second field, SCN became detectable after 2003 during the monoculture in nonfumigated plots and lagged in fumigated plots; both treatments had low levels of SDS. Exploiting the suppressiveness of the first field could allow for biological control of SDS and SCN in soybean production.

Pythium spp. Isolated from Bermudagrass During Overseed Transitions in Florida and Pathogenicity of Pythium irregulare on Poa trivialis.

Pythium spp. were isolated from nonoverseeded bermudagrass and from bermudagrass overseeded with cool-season turfgrass species from October 2000 to July 2001 from two sites in Florida. Pythium spp. were isolated from nonoverseeded and overseeded, and fumigated and nonfumigated, bermudagrass plots from October 2001 to July 2002 from one of the two sites. The vast majority of isolates of Pythium spp. were obtained from the bermudagrass, rather than the cool-season turfgrass species (Poa trivialis or Lolium perenne) used as overseed. In the first year at the Ft. Lauderdale site, Pythium graminicola dominated (91% of isolates obtained). In the first year at the Gainesville site, P. graminicola (56% of isolates) and P. irregulare (36%) dominated; however, after fumigation and replanting, P. graminicola comprised only 11% of all isolates. P. irregulare comprised 30% of all isolates, but was found only in nonfumigated plots. A different species, P. ultimum var. ultimum, not isolated in the first year, was recovered (34% of isolates) from the replanted field in February and March. In preemergence pathogenicity tests, three of four isolates of P. irregulare were moderately to highly pathogenic on Poa trivialis, but not on L. perenne, and isolates of Pythium graminicola and P. ultimum var. ultimum were not pathogenic on either turfgrass species. Pathogenic Pythium spp. may survive from season to season on bermudagrass and, under favorable conditions, may cause damping-off or blight on the overseeded cool-season turfgrass.

Pearl millet as a rotation crop with flue-cured tobacco for control of root-lesion nematodes in Ontario

In Ontario, flue-cured tobacco (Nicotiana tabacum L.) growers routinely fumigate their soils to control root-lesion nematodes (Pratylenchus penetrans Cobb). Studies suggest that planting pearl millet (Pennisetum glaucum L.) as a rotation crop may control the nematodes; winter rye (Secale cereale L.), the crop commonly grown in rotation with tobacco, is susceptible to the nematodes. In 2002, plots of forage millet (var. CFPM 101), grain millet (var. CGPMH-1) and winter rye were established at three sites and in 2003, tobacco was grown at these sites in non-fumigated plots where the rotation crops grew, except for a winter rye/fumigation treatment. The goal was to evaluate P. penetrans populations and tobacco yield and quality in the millet rotations relative to the traditional rye-fumigation system. In 2002, initial P. penetrans populations assessed in the spring were similar for all treatments; however, the final populations and consequently the ratios of final populations/initial populations (Pf/Pi) consistently differed among treatments. The Pf/Pi ratios ranged from 0.07 to 0.79 for forage millet, from 0.18 to 0.94 for grain millet and from 2.33 to 21.65 for rye. In 2003, P. penetrans populations in tobacco plots previously cropped to either type of millet were comparable with those detected in fumigated winter rye plots. Compared with tobacco following rye without fumigation, tobacco yield was 14 to 51% higher following rye with fumigation, 19 to 70% higher following forage millet and 18 to 65% higher following grain millet. Grade index was unaffected. Results indicate that pearl millet offers an alternative to fumigants for the suppression of P. penetrans population and provides equivalent gross returns. Key words: Pennisetum glaucum, rotation crop, Nicotiana tabacum

Performance of Containerized and Bare-root Transplants with Soil Fumigants for Florida Strawberry Production

Field studies were conducted in three Florida locations (Bradenton, Gainesville, and Quincy) during 1998-99 and 1999-2000 to: 1) compare the performance of two transplant systems under diverse MBr alternative programs in `Chandler' strawberry (Fragaria ×ananassa), and 2) determine the efficacy of these treatments on soilborne pest control in strawberry. Fumigant treatments were: 1) nonfumigated control, 2) methyl bromide plus chloropicrin (MBr + Pic) at a rate of 350 lb/acre, 3) Pic at 300 lb/acre and napropamide at 4 lb/acre, 4) 1,3-dichloropropene (1,3-D) plus Pic at 35 gal/acre and napropamide at 4 lb/acre, 5) metam sodium (MNa) at 60 gal/acre and napropamide at 4 lb/acre, and 6) MNa followed by 1,3-D at 60 and 12 gal/acre and napropamide at 4 lb/acre, respectively. Strawberry transplants were either bare-root or containerized plugs. There were no significant fumigant by transplant type interactions for strawberry plant vigor and root weight per plant, whereas ring nematode (Criconema spp.) and nutsedge (Cyperus rotundus and C. esculentus) populations, and total marketable fruit weight were only infl uenced by fumigant application. The nonfumigated plots had the lowest strawberry plant vigor and root weight per plant in all three locations. In most cases, plant vigor and root biomass per plant increased as a response to any fumigant application. With regard to the transplant type, bare-root transplants had similar plant vigor as plugs in two of the three locations. Fumigation improved nutsedge and ring nematode control. All fumigants had higher early and total marketable yield than the nonfumigated control, whereas transplant type had no effect on total fruit weight.

Relationship between Yield Potential and Percentage Yield Suppression Caused by the Southern Root‐Knot Nematode in Cotton

Currently available cotton (Gossypium hirsutum L.) cultivars support significant reproduction of the southern root‐knot nematode [Meloidogyne incognita (Kofoid & White) Chitwood], but they have not been evaluated for differing levels of yield suppression (tolerance) to this nematode. If nematode tolerant (low yield suppression) but susceptible (high nematode reproduction) cotton cultivars can be identified, they could be grown rather than intolerant cultivars to reduce yield loss. The objective of this study was to evaluate a collection of M. incognita–susceptible cotton cultivars for tolerance to parasitism by this nematode. The yield potential and percentage yield loss to M. incognita were measured in 12 genotypes in 2002 and 2003 by comparing yields in 1,3‐dichloropropene–fumigated and nonfumigated plots. The percentage yield suppression caused by M. incognita differed among cotton genotypes in both 2002 and 2003. Yield suppression ranged from 18.0 to 47.3% in 2002 and from 8.5 to 35.7% in 2003. Though significant levels of tolerance were measured in our study, 2 yr of data on percentage yield suppression show that tolerance is not consistently related to specific cultivars in the absence of nematode resistance: susceptible cultivars did not consistently express tolerance, but resistant germplasm did. Thus, it appears unlikely that cotton cultivar selection for tolerance to M. incognita can be utilized to minimize yield suppression. Regression analysis based on the 2 yr of field data revealed a relationship in which percentage yield suppression caused by M. incognita increased linearly as yield potential increased. Because the absolute and percentage losses to nematodes increase as yield potential increases, nematode management becomes increasingly important and beneficial in cotton.

Depth distribution of Rotylenchulus reniformis under crops of different host status and after fumigation

Abstract Population densities of Rotylenchulus reniformis were investigated in 15 cm horizons from the surface to 120 cm deep in field plots under fallow, grain sorghum (cv. Asgrow 571), and susceptible (cv. DP6880RR) or resistant (cv. HY798) soybean. In 2000, population densities were monitored in non-fumigated plots and in plots fumigated pre-season with 1,3-dichloropropene (1,3-D) at 38 cm depth. Fallow, grain sorghum and resistant soybean reduced the incidence of R. reniformis down to 120 cm in comparison to susceptible soybean. In 2001, population densities were monitored under cotton in these plots and in additional plots that had grown susceptible soybean cv. Vernal or resistant cv. Padre in 2000 and that were fumigated before the 2001 crop with 1,3-D at depths of 0-60, 60-120, or 0-120 cm. In 2000 non-fumigated plots, cotton fibre yields were increased by an average 35% after fallow, grain sorghum or resistant soybean compared to those after susceptible soybean. In the 2000 fumigated plots, cotton fibre yields were increased 38% after grain sorghum and resistant soybean compared to those after susceptible soybean or fallow. In plots cropped previously to susceptible soybean, fumigation at 60-120 cm deep increased cotton fibre yields by 68% compared to the non-fumigated control. Population densities in the 0-120 cm horizon were a more accurate predictor of plant damage than those at 0-30 cm. The value of R. reniformis resistant soybean cultivar-cotton crop sequences was confirmed and the importance of their effects on deep-occurring populations of R. reniformis demonstrated.

EFFECT OF FUMIGATION AND COMPOSTED YARD WASTE ON STRAWBERRY YIELD, PLANT AND ROOT QUALITY

Dormant 'Earliglow' strawberry plants (Fragaria x ananassa Duch) were established in a replicated split plot factorial design in 1994. Main plots were fumigated or non-fumigated. The four subplots consisted of pre-plant applications of composted yard waste (CYW) applied to the row and incorporated (subsurface), surface-applied (surface), incorporated plus surface applied (subsurface + surface) and a control. Berries were harvested in 1995 through 1997 and analyzed for total weight (yield) and percent of culls. Plants were excavated in 1997 and analyzed for plant weight and percent black roots. Yields and percent culls varied among years. In 1995, fumigated plots had more yield than non-fumigated plots. Fumigated subsurface CYW alone had lower fresh and dry plant weight than the fumigated control. Cumulative yields for CYW plots had 9 to 17% more total weight for fumigated and 6 to 15% for non-fumigated than the controls. Fumigation and surface applied CYW had a lower percent of black roots than the control.

Reduction of the available phosphorus pool in field soils growing maize genotypes with extensive mycorrhizal development

Arbuscular mycorrhizal fungi (AMF) have a large enhancing effect on the P uptake capacity of host plants, which could make possible the production of high crop yields on soil with reduced level of available P, or could help reduce the P level in rich soils, thereby reducing the risk of P loss to the environment. A field experiment was conducted in Ste-Anne-de-Bellevue, Quebec, Canada, on a loamy sand in 1997 and a fine sandy loam in 1998 to assess the impact of indigenous AMF-maize hybrid combinations on soil available P level. The experiment had three factors organized in a split-split plot design. There were two soil fumigation levels (fumigated and non-fumigated) randomized in the main plots, three P fertilizer rates (0, 60, and 120 kg ha-1) randomized in the sub-plots and three maize (Zea mays L.) hybrids with contrasting genotypes [two newly developed leafy hybrids, Leafy normal stature (LNS) and Leafy reduced stature (LRS), and a commercial hybrid, Pioneer 3979 (P3979)], which were randomized in the sub-sub-plots. Soil extractable P, plant P content, plant dry mass, root colonization with AMF and extraradical hyphae were determined at the 6-leaf, 10-leaf, tasselling and silking stages of maize, and grain yield and total P in maize were determined at harvest. Soil fumigation to reduce AMF and P fertilization reduced the amount of indigenous mycorrhizal development in maize hybrids. The growth of LNS, the most mycorrhizae-dependent hybrid, was more supressed by fumigation than the growth of the other two hybrids. When the soil P level was low, plant dry mass, grain yield and total P content of LNS were higher in the non-fumigated plots than in the fumigated plots. Fumigation had a significant but smaller influence on soil extractable P level than on plant P uptake and growth. Soil extractable P was lower in non-fumigated plots than in fumigated plots from silking to the end of the growing season in 1997, only in non-fertilized plots growing LNS. Extraradical hyphae density was positively correlated with maize P uptake and negatively correlated with soil extractable P. This suggested that soil extractable P can be reduced through AMF-enhanced plant P uptake when soil available P conditions and host plant genotype are favourable to mycorrhizal development, and when P uptake enhancement is large. Key words: Arbuscular mycorrhizal fungi, maize hybrids, soil extractable P, P uptake, extraradical hyphae, root colonization, mycorrhizal dependency

The effects of soil fumigation on pine seedling production, weeds, foliar and soil nutrients, and soilborne microorganisms at a south Georgia (U.S.A.) forest tree nursery

Pine seedling production and pest problems were evaluated in plots fumigated with methyl bromide and nonfumigated plots over a 6-year period at a Georgia nursery. Fumigation increased bed densities for loblolly pine (Pinus taeda L.) in 1996 and slash pine (Pinus elliottii Engelm. var. elliottii) in 1998; differences were not observed between treatments in other years. The root collar diameter, height, and root and shoot masses of loblolly pine seedlings were often greater in fumigated plots during the first 3 years. Morphological characteristics rarely differed between treatments for slash pine. The primary pest problem was nutsedge (Cyperus spp.); most other weeds were controlled with herbicides used operationally at the nursery. Plant-parasitic nematode populations did not increase over time and were not a problem. Although Fusarium and Pythium spp. were more common in soil and on roots in nonfumigated plots, evidence of disease was rare. Fumigation increased the abundance of and changed the composition of Trichoderma spp. in soil and on roots. Soil manganese and iron, and foliar manganese, phosphorus, and nitrogen were greater in the fumigation treatment in some years. A better understanding of the risks of soilborne diseases may facilitate the development of pest management programs that are more cost-effective.

Evidence for biological nature of the grape replant problem in California

A bioassay was developed to investigate causes of grape replant problems under controlled conditions. Soils were collected from methyl bromide-fumigated and non-fumigated plots at a site cleared from a 65-year-old grape vineyard (Vitis vinifera cv. Thompson seedless) at Parlier, CA. Subsamples of the non-fumigated soil were either left non-treated, subjected to autoclaving (twice 45 min), or heating at 40, 50, 60, 70, 80 or 90 °C for 30 min. Subsequently, the samples were placed in 120-mL pots, planted with rooted hardwood grape cuttings (V. vinifera, cv. Carignane) and placed in a greenhouse or growth chamber. Three months after transplanting, vines from non-treated or 40 °C-treated soil had lower shoot weights and densities of healthy lateral roots than vines from the other treatments. Pythium spp. were isolated from 45 to 55% of the plated root segments from vines grown in non-treated, or soil that had been heated at 40 or 50 °C but were not detected in roots from soil given other treatments. Egg masses of root-knot nematode, Meloidogyne spp., were produced on roots from non-treated or heated at 40 °C soil, but no egg masses were detected on roots of the other treatments. In another test with the same soils, remnant roots from non-fumigated or pre-plant methyl bromide-fumigated soil were extracted and amended to non-fumigated soil, soil from fumigated field plots, soil fumigated in a small container, or autoclaved potting mix. The transfer of old vine roots from non-fumigated field soil resulted in incidence of Pythium spp. on grape assay roots, but there was no measurable effect of the transfer on growth and health of the bioassay plant roots. The results of the bioassays indicate that grape replant problem at the California site had biological causes. The bioassay approach may aid in future determinations of the etiology of grape replant problems.

First Report of Fusarium solani Causing Stunt on Lisianthus.

Fusarium solani Mart. (Sacc.) is the causal agent of stem rot and damping-off of lisianthus (Eustoma grandiflorum (Raf.) Shinn.) (1). Since the end of the 1980s, when this flower crop was introduced in Argentina, it has been affected by a basal stem rot (2). A previously undescribed disease was observed in 100% of the greenhouses in the Buenos Aires Province that grow lisianthus. Symptoms that developed after seedlings were transplanted included stunting, shortened internodes with reduced stem diameter, and small narrow leaves that were a dull green color. Some affected plants turned yellow-brownish and died 2 to 3 months after transplanting. Other plants recovered but produced low quality flowers later than normal. A third group of plants remained stunted (5 to 10 cm high) until the last flower harvest (about 8 to 10 months). F. solani was consistently isolated from basal stems and roots of diseased plants. For pathogenicity tests, inoculum was produced by culturing the fungus for 10 days in petri dishes containing sterile moistened rice. Inoculum was air dried, crushed, and mixed with soil that had been autoclaved at 112°C for 40 min on each of two consecutive days. The propagules in the soil were estimated by soil plate dilutions on the Nash & Snyder-PCNB medium at a ratio of about 104 CFU/g soil. Twenty plants of each cultivar Echo White and Echo Blue, whose roots had been pruned, were planted in both infested and noninfested soil. After about 40 days, stunting was observed in 85% of the inoculated plants, while controls remained asymptomatic. F. solani was reisolated from symptomatic plants, thus fulfilling Koch's postulates. A test also was conducted in a commercial greenhouse that produced lisianthus for several years, in which healthy plants were planted in three plots fumigated with methyl bromide and in three nonfumigated plots. The mean cfu/g soil of F. solani in the methyl-bromide treated plots was 5 × 102 and 1.6 × 104 CFU/g in the nontreated plot. After 120 days, the incidence of stunting in the treated plots was 0.6 and about 88% in the control plots. F. solani was recovered from symptomatic plants. Because disinfestation of soil is generally practiced in flower production, stunted plants are limited and can be confused with root problems. This is the first report of F. solani causing stunt on lisianthus.

Effects of preplant inoculation of apple (Malus domestica Borkh.) with arbuscular mycorrhizal fungi on population growth of the root-lesion nematode, Pratylenchus penetrans

Six species of arbuscular mycorrhizal (AM) fungi (Glomus aggregatum, G. clarum, G. etunicatum, G. intraradices, G. mosseae and G. versiforme) were evaluated, in three greenhouse experiments, for their effects on reproduction of the root-lesion nematode, Pratylenchus penetrans, and growth of Ottawa 3 apple rootstock. Glomus mosseae increased total dry weights of nematode-inoculated and non-inoculated rootstock in all three greenhouse experiments, and G. intraradices increased dry weights in two of three greenhouse experiments. Plants inoculated with G. mosseae generally supported fewer P. penetrans per gram of root than plants inoculated with other AM fungi, but did not differ significantly from the controls in any greenhouse experiment. Colonization of roots by AM fungi was reduced by P. penetrans at initial inoculum densities greater than 250 nematodes/L soil. In field trials, preplant inoculation with either G. intraradices or G. mosseae increased rootstock growth and leaf concentrations of P, Mg, Zn and Cu in fumigated plots but not in non-fumigated plots, indicating that colonization by native AM fungi in non-fumigated plots may have been sufficient for adequate nutrient acquisition. The abundance of vesicles and arbuscules was greater in roots of plants inoculated with AM fungi before planting than in roots of non-inoculated plants, in both fumigated and non-fumigated plots. P. penetrans per gram of root and per 50 ml soil were significantly lower for G. mosseae- inoculated plants than for non-inoculated plants in fumigated soil but not in non-fumigated soil.

Performance of Strawberry Cultivars on Fumigated and Nonfumigated Soil in Florida

Five strawberry (Fragaria ×ananassa Duch.) cultivars in the 1995-96 season and four cultivars in the 1996-97 season were grown in the annual hill plasticulture system, with and without preplant soil fumigation (98% methyl bromide/2% chloropicrin at a rate of 240 lb/acre [269 kg·ha-1]). These trials were established on land that had been cropped with strawberries for 20 years. Significant cultivar by fumigation interactions were not detected for either yield or average fruit weight. Plants grown in nonfumigated soil produced 54% and 68% of the yield obtained from the plants grown in fumigated soil in 1995-96 and 1996-97 respectively, and the average fruit weight from plants grown in nonfumigated soil was also reduced, compared to that of plants grown in fumigated soil. Plant mortality was ≤3% in the nonfumigated plots. These results indicate that strawberry productivity in Florida can be substantially reduced by growing plants in soil that has not been fumigated prior to planting, even in the absence of lethal pathogens.