Milton E. McGiffen

Methyl iodide, an ozone-safe alternative to methyl bromide as a soil fumigant

Methyl iodide was tested as a possible replacement for methyl bromide as a soil fumigant due to the scheduled removal of methyl bromide from the market. Methyl iodide is a better methylating agent than methyl bromide; it is rapidly destroyed by UV light and therefore unlikely to be involved in stratospheric ozone depletion. In laboratory and field trials, we tested methyl iodide alone or in comparison with methyl bromide for effectiveness in controlling the fungi Phytophthora citricola, P. cinnamomi, P. parasitica, and Rhizoctonia solani; the nematode Heterodera schachtii; and the plants Abutilon theophrasti, Chenopodium album, C. murale, Convolvulus arvensis, Cyperus rotundus, Poa annua, Portulaca oleracea, and Sisymbrium irio. In addition, we compared methyl iodide for biocidal effectiveness with seven other alkyl iodides. In both laboratory and field trials, when compared at equivalent molar rates, methyl iodide was equal to or better than methyl bromide in controlling the tested soilborne plant pathogens and weeds. When compared with other alkyl iodides, methyl iodide was the most effective fumigant.

Effect of cover crop and management system on weed populations in lettuce

Abstract Changes in cropping systems can significantly impact pest population dynamics and management. Field experiments were conducted from 1998 to 2000 to study the effect of summer cover crop and in-season management system on weed infestations in lettuce ( Lactuca sativa L.). The experiment was a factorial with summer cover crop and management system as the two factors. Cover crops were cowpea [ Vigna unguiculata (L.) Walp.] incorporated into the soil in the fall (CWPI), cowpea used as mulch in the fall (CWPM), sudangrass ( Sorghum vulgare ) incorporated into the soil in the fall (SDNG), and dry fallow (DFLW). Management systems were organic (ORG), integrated crop management (ICM), and conventional (CNV). In general, there was no interaction between cover crop and management system. Weed species richness was greatest early in the growing season and was affected by cover crop. Fewer species emerged in CWPM plots in general but perennial species became important with time. The DFLW showed greater weed populations than all cover crop treatments. Weed populations were similar in all management systems in 1999. However, in 2000, there was better weed suppression in the ORG and ICM systems than in the CNV system. CWPI significantly increased lettuce yield and DFLW yield declined the second year. Yield under the ORG system was low in 1999 and equivalent to that in the CNV and ICM systems in 2000. These results indicate that a prior summer cover crops can improve both conventional and organic vegetable production systems.

Evaluation of methyl iodide as a soil fumigant in container and small field plot studies

Methyl iodide was evaluated as a soil fumigant as a potential replacement for the widely used soil fumigant methyl bromide. In container trials, methyl iodide was significantly more effective than methyl bromide against the plant parasitic nematodes Meloidogyne incognita, Heterodera schachtii and Tylenchulus semipenetrans and the plant pathogenic fungus Rhizoctonia solani. In small field plots, soil populations of root-knot nematodes were no longer detected after methyl iodide fumigation at an application rate of 112 kg ha-1. However, after growing a susceptible lima bean host for two months, substantial root-knot galling occurred, while Rhizobium nodulation was absent. At 168 kg ha-1 of methyl iodide, root-knot galling was reduced to less than 1%, and no Pythium propagules were recovered on selective detection media. These efficacy data support the conclusion that methyl iodide is a likely candidate for replacing methyl bromide as a soil fumigant.

Efficacy of methyl iodide and synergy with chloropicrin for control of fungi

Efficacy of soil fumigation with methyl bromide and methyl iodide against Botrytis cinerea, Colletotrichum gloeosporioides, Fusarium oxysporum, Gliocladium virens, Phytophthora citricola, Phytophthora citrophthora, Pythium ultimum, Rhizoctonia solani and Verticillium dahliae was determined in laboratory experiments in closed fumigation chambers. Pythium ultimum was the most sensitive fungal species with EC 50 values for methyl bromide and methyl iodide of 15.5 and 8.6 μM, respectively. R solani was the least sensitive with EC 50 values of 253.4 and 161.4 μM for methyl bromide and methyl iodide, respectively. Relative potency ([methyl bromide]/[methyl iodide]) values ranged from 5.2 for P citricola to 1.5 for F oxysporum. Methyl iodide was 2.7 more efficacious than methyl bromide averaged over all fungal species. Methyl bromide/chloropicrin and methyl iodidelchloropicrin applied jointly were 2.2 and 2.8 times more efficacious, respectively, against F oxysporum than when the compounds were applied singly. Combining methyl bromide and methyl iodide with chloropicrin resulted in a significant synergistic increase in activity against F oxysporum.

Effect of soil physical factors on methyl iodide and methyl bromide

Production and importation of methyl bromide is scheduled to be banned by 2001. Methyl iodide was evaluated as a possible replacement soil fumigant. The effects of soil moisture, temperature, soil texture and fumigation time on the efficacy of methyl iodide for the control of two common weeds, Abutilon theophrasti and Lolium multiflorum, were characterized and compared with those of methyl bromide. The optimal soil moisture for methyl iodide to kill both weed species in sandy soils was 14% water content (w/w). Greater efficacy was obtained when the temperature during fumigation was above 20°C. Compared to methyl bromide, the efficacy of methyl iodide was more consistent in different soils. Time to 100% mortality of weeds was 24 h for methyl iodide fumigation and 36 h for methyl bromide when 200 μM of fumigant was used. On a molar basis methyl iodide was consistently more effective than methyl bromide across the range of environmental factors tested. In terms of application technology and spectrum of activity, methyl bromide can be directly replaced by methyl iodide.

Comparison of methods to estimate weed populations and their performance in yield loss description models

Abstract Accurate weed population estimation and yield loss prediction are important components of integrated weed management. Field experiments using Italian ryegrass as a weed in broccoli were conducted from 1994 to 1997 to compare weed density to other methods of weed population estimation, to evaluate the performance of weed population estimates in yield description models, and to study the affect of environmental variability on the predictive ability of models. A strong linear relationship was obtained between Italian ryegrass density and direct leaf area (r2 = 0.60 to 0.99). For Italian ryegrass, density and estimates of canopy from the optical device (crosswire device) had a hyperbolic relationship with high coefficients of determination (r2 > 0.72). Both direct leaf area and canopy estimates described broccoli yield as well as or better than Italian ryegrass density. The Li-Cor LAI-2000 Plant Canopy Analyzer (PCA) provided poor estimates of Italian ryegrass population (r2 from 0.00 to 0.63) that failed to describe broccoli yield. No relationship was observed between estimates of light interception through the plant canopy obtained with the Li-Cor LI-191-S Line Quantum Sensor (LQS) and either Italian ryegrass density or broccoli yield. The low performance of the PCA and lack of performance of the LQS were likely due to the smaller size of the plants and larger gaps in the plant canopy caused by wide bed spacing. At similar densities, Italian ryegrass competition with broccoli was stable from year to year. Under high Italian ryegrass density, water supply affected competition. This may limit construction of robust yield prediction models, especially in areas where water is mainly from rainfall. Nomenclature: Broccoli, Brassica oleracea L. var. botrytis; Italian ryegrass, Lolium perenne L. LOLPE.

Simple Approaches to Improve Restoration of Coastal Sage Scrub Habitat in Southern California

ABSTRACT Much of the coastal sage scrub habitat in Southern California that existed prior to European settlement has been developed for human uses. Over the past two to three decades, public agencies and land conservation organizations have worked to acquire some of the remaining lands for preservation. Many of these lands are degraded by past intensive livestock grazing, farming, and frequent fires, and the native flora has been replaced by weedy, exotic annual grasses and forbs, mostly of Mediterranean origin. Restoration of native flora is challenging and there are few successful examples to provide guidance on effective methods. Cost is also an important and prohibitive factor. Competition from weeds is one of the most difficult impediments to establishing native vegetation, which often persists in the seedbank. We compared annual applications of the nonselective herbicide glyphosate over multiple years, followed by a final year with the grass-specific fluazifop, as a simple, low cost method of reducing the exotic seedbank sufficiently to allow native vegetation to establish. This approach was combined with seeding native forbs, herbaceous perennials, and shrubs in one half of each treatment plot. Herbicide treatments were made in the spring each year from 2006 to 2010, and were combined with weed trimming in 2010 and 2011 to remove exotic forb inflorescences, and raking to remove litter. In 2010, native plant cover in herbicide-treated plots was about 50%, consisting of 43 species, compared to <5% cover in the control plots. Most of the native plants came from the existing seedbank, and very few from the seed mix. A cost analysis showed that a once-yearly herbicide treatment was as effective as one application plus spot spraying or hand inflorescence trimming, and is more cost-effective than hand weed control and raking for restoration.

Index for potato nitrogen status diagnosis in a three-phase hydroponic system

Abstract Up to now, there has not been any research that has tried to establish nitrogen (N) status diagnosis in basic seed potatoes, propagated by sprout in a three-phase hydroponic system. The objective of this study was to develop a potential index to be used in the diagnosis of N status in potato, using a three-phase hydroponic system, with the potato cultivar Agata. The treatments consisted of four N concentrations: 0, 7.8, 13 and 26 mmol L−1. The experiment was conducted in a randomized block design with 12 replications in a non-acclimatized greenhouse. It was evaluated in a destructive and non-destructive way, and in real-time or space–time, 18 biometric characteristics of the reference leaf and of plant organs besides other variables related to the fourth leaf green color. Among the non-destructive characteristics measured, length, and diameter of the internodes and thickness of the fourth leaf showed the highest correlation coefficient (p < .01) with the N content in dry matter of the fourth leaf. This correlation can be utilized for the diagnosis of the N status of potato in a three-phase hydroponic system. Furthermore, we propose a value table to classify the N status of the plant based on the main evaluated characteristics.