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Mathieu Ngouajio

Over the last century, climate change, adoption of new regulations, and changes in cropping systems have significantly impacted weed and pest management in horticultural crops. The objective of this workshop was to provide a critical review of major changes and discuss current and future trends for weed and pest management. Speakers touched on a broad range of topics including climate change and disease dynamics, the use of disease resistance inducers, soil management for pest management, and the role of allelopathy in weed management. Major recommendations included 1) increased grower education related to the impact of climate change on plant diseases; 2) more research directed towards a better understanding of the interaction of plant–pathogen–inducer; 3) use of organic soil amendments, cover crops, crop rotations, and resistant cultivars to enhance the weed and disease suppressive effect of soils; and 4) enhancement of allelochemical production and subsequent weed suppression through conventional breeding and molecular techniques.

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Mathieu Ngouajio and Jeremy Ernest

Weed control is one of the benefits associated with the use of plastic mulches used for vegetable production. The mulches decrease light transmission and prevent development of most weed species. Plastics chemistry has developed films varying in their ability to reflect, absorb, and transmit light. Laboratory and field experiments were conducted to 1) measure light transmitted through colored mulches, 2) evaluate weed populations under each mulch type, and 3) determine if light transmission could be used as an indicator for weed populations in the field. The polyethylene mulches were black, gray, infrared transmitting brown (IRT-brown), IRT-green, white, and white-on-black (co-extruded white/black). On average, 1%, 2%, 17%, 26%, 42%, and 45% light in the 400 to 1100 nm range was transmitted through the black, white/black, gray, IRT-brown, IRT-green, and white mulches, respectively. In field experiments, density and dry biomass of weeds growing under the mulches were evaluated. The white mulch had the highest weed density with an average of 39.6 and 155.9 plants/m2 in 2001 and 2002, respectively. This was followed by the gray mulch, with 10.4 and 44.1 weed seedlings/m2 in 2001 and 2002, respectively. Weed density was <25 plants/m2 with the other mulches in both years. Weed infestation was correlated with average light transmission for white, black, white/black, and gray mulches. However, both light quantity and quality were necessary to predict weed infestations with the IRT mulches. Weed infestation under the IRT mulches was better estimated when only wave lengths in the photosynthetically active radiation range (PAR; 400 to 700 nm) were considered. Low weed pressure and high light transmission with the IRT mulches would make them appropriate for use in areas where both weed control and soil warming are important factors.

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Mathieu Ngouajio and Jeremy Ernest

In temperate regions, the vegetable growing season is short and plastic mulches are usually left in the field for an entire year when used for double cropping. This work was conducted to study the effect of weathering on the physical, optical, and thermal properties of plastic mulches during double cropping. The design was a randomized complete block with four replications. The mulches were black, grey, infrared transmitting brown (IRT-brown), IRT-green, white, and white-on-black (co-extruded white/black). Tomato was grown the first year and cucumber the following year. The grey mulch degraded substantially during double cropping (only 40% of bed was covered the second year) and showed an increase in light transmission and a decrease in heat accumulation (degree-days). The black, whiteon-black, white, IRT-brown, and IRT green mulches showed less degradation with 93%, 91%, 85%, 75%, and 61% soil cover, respectively. However, their soil warming ability was significantly reduced. These mulches could be used for double cropping to suppress weeds and to reduce inputs associated with plastic purchase, laying, and disposal. However, they may not provide adequate soil warming early in the season for the second crop.

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Mohan Selvaraj* and Mathieu Ngouajio

The inclusion of cover crops into cropping systems may influence soil microbial activity which is crucial to sustained crop production. A study was conducted to measure short term effects of summer and winter cover crops on soil microbial biomass carbon (MBC) in a cucumber-tomato rotation system. The experiment was established in Summer 2002 as a factorial of summer cover crops (planted either as fallow or after harvest of cucumbers) and winter cover crops (planted in September). The design was a split-block with four replications. The main plot factor was summer cover crop and consisted of five treatments; sorghum sudangrass fallow (SGF), cowpea fallow (CPF), sorghum sudangrass after cucumber (SGC), cowpea after cucumber (CPC) and bareground fallow (BGF). The sub-plot factor was winter cover crop and consisted of three treatments including cereal rye (CR), hairy vetch (HV) and bareground (BG). In spring of 2003, soil samples were collected in each treatment at 30 days before (30 DBI), 2 days after (2 DAI) and 30 days after (30 DAI) cover crop incorporation. MBC was measured using the chloroform fumigation-incubation method. Both summer and winter cover crops affected soil microbial activity. MBC in the summer cover crop treatments at 30 DBI was 47.7, 51.4, 49.2, 43.7 and 42.5 μg·g-1 soil for SGF, CPF, SGC, CPC and BGF, respectively. At 30 DAI, 113.1, 88.9, 138.5, 105.6, and 109.3 μg·g-1 soil was obtained in SGF, CPF, SGC, CPC, and BGF plots, respectively. Soil MBC was similar at 2 DAI in the summer cover crop treatments. Among winter treatments MBC was similar at 30 DBI and 30 DAI, but significant at 2 DAI with values of 62.8, 53.3, 59.3 μg·g-1 soil for CR, BG, and HV, respectively.

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Ajay Nair and Mathieu Ngouajio

The area of organic production has registered a steady increase over past recent years. Transitioning to organic production is not straightforward and often includes a steep learning curve. Organic growers have to develop strategies to best manage nutrients, pests, and crop growth and yield. Additionally, in regions with temperate climate like the Great Lakes region, weather (especially temperature and solar radiation) plays an important role in crop productivity. Growers routinely use compost for nutrient provisioning and rowcovers for insect exclusion and growth enhancement. The objective of this work was to study the combined effect of rowcovers (with different light transmission) and compost organic cucumber (Cucumis sativus L.) growth and microclimate. Plots were assigned to three rowcover treatments (60% light transmission, 85% light transmission, and uncovered) and two amendment treatments (compost and no compost) in a split-plot factorial design. Data were collected for ambient air and soil temperature, photosynthetically active radiation (PAR), relative humidity, plant growth characteristics, and yield. Rowcovers modified crop microclimate by increasing air and soil temperature and decreasing PAR. There was a marked increase in the growing degree-day accumulations under rowcovers when compared with uncovered treatment. The impact of rowcovers on plant growth was significant. Use of rowcovers increased vine length, flower count, leaf area, leaf count, plant biomass, and total marketable yield. Use of compost in conjunction with rowcovers enhanced the rowcover effect. With the use of compost, there were not many significant differences in plant growth characteristics between rowcover materials; however, as expected, rowcover with 60% transmission was able to trap more heat and reduce light transmission when compared with rowcover with 85% transmission. This study clearly shows the importance of organic amendments, especially compost, in organic vegetable production. Applications of compost enhanced crop growth and also led to higher marketable yields. Results of this study suggest additive effects of rowcover and compost application on organic cucumber production.

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Guangyao Wang and Mathieu Ngouajio

The effects of two cover crops [cereal rye (Secale cereale L.) and oat (Avena sativa L.)], four tillage systems [no tillage (NT), strip tillage (ST), conventional tillage with cover crops incorporated (CTC), and conventional tillage without cover crop (CTN)], and three pre-emergence herbicide rates (full rate, half rate, and no herbicide) on pickling cucumber (Cucumis sativus L.) growth and production, weed populations, and the incidence of pythium fruit rot were studied. Weed infestations, cucumber establishment, and cucumber leaf chlorophyll content were similar between the rye and oat treatments. However, the oat treatment had higher cucumber fruit number and weight and a lower percentage of cucumber fruit infected with Pythium spp. compared with the rye treatment. The NT and CTC systems reduced cucumber stand and leaf chlorophyll content, but had equivalent cucumber fruit number and weight compared with CTN. The NT and ST had lower weed biomass and weed density than CTN and CTC. The NT also reduced the percentage of cucumber fruit affected with pythium compared with CTN and CTC. Reducing the pre-emergence herbicide rate by half did not affect weed control or cucumber fruit yield compared with the full rate. However, weeds escaping herbicide application were larger in the half-rate treatment. The experiments indicate that with the integration of cover crops and conservation tillage, it is possible to maintain cucumber yield while reducing both herbicide inputs (by 50%) and the incidence of fruit rot caused by Pythium spp. (by 32% to 60%).

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Husrev Mennan and Mathieu Ngouajio

Weed management is critical in hazelnut (Corylus avellana) production. Weeds reduce nutrient availability, interfere with tree growth, and reduce hand-harvesting efficiency. Field experiments were conducted from Fall 2006 to 2010 to test effects of brassica (Brassica sp.) cover crops and hazelnut husk mulch as alternative weed management strategies in hazelnut. The cover crop treatments consisted of rape (Brassica napus), field mustard (Brassica rapa), oriental mustard (Brassica juncea), and fallow with no cover crop. Hazelnut husk was surface-applied at two thicknesses, 5- and 10-cm-thick layer. Dry biomass production by the cover crops was relatively consistent among years with oriental mustard producing the most biomass. Throughout the growing seasons, the cover crops reduced weed density, weed dry weight, and the number of weed species when compared with the fallow treatment. The most effective cover crop at suppressing weeds was oriental mustard. Hazelnut husk applied as a 10-cm-thick layer on the ground was highly effective at controlling weeds up to 180 days after application and reduced total weed dry weight by 83% at the end of the season. Our findings indicate that brassica cover crops or hazelnut husk may help control annual weed species in hazelnut orchards during early summer. However, these strategies should be combined with other methods like chemicals or cultivation for adequate weed management.

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Victoria J. Ackroyd and Mathieu Ngouajio

Field and laboratory bioassay studies were conducted to determine the impact of Brassicaceae cover crops on cucurbit germination percentages and stand counts. A 2-year field study in southwestern Michigan examined the effect of oilseed radish (Raphanus sativus var. oleiferus), oriental mustard (Brassica juncea), and yellow mustard (Sinapis alba) green manures on muskmelon (Cucumis melo Group reticulatus) stand. All three cover crops reduced direct-seeded muskmelon stand count as well as transplant survival. Stand count for direct- seeded muskmelon was greater than 85% for control and methyl bromide treatments and less than 41% for cover crop treatments. Oilseed radish had the greatest effect with 0% muskmelon stand in both years. The use of transplants improved muskmelon stand establishment. However, stand count (less than 45% to 50%) was still unacceptable. In bioassays, muskmelon, cucumber (Cucumis sativus), and honeydew melon (Cucumis melo Group inodorus) seeds were exposed to either non-lyophilized or lyophilized root and shoot aqueous extracts of oilseed radish. Germination percentages and radicle elongation measurements showed both extracts impacted all three crops to varying degrees. Muskmelon germination was least sensitive to the extracts, followed by cucumber, then honeydew. Cucumber and muskmelon root growth was equally inhibited by non-lyophilized shoot extract, while honeydew growth was mildly stimulated at 5% and 12.5% concentrations. Overall, non-lyophilized root extract showed stronger inhibition on seed germination than non-lyophilized shoot extract, while the reverse was true of lyophilized extracts. In general, non-lyophilized extracts had far greater impact on germination percentages and radicle elongation than lyophilized extracts. These results suggest species and tissue dependent toxicity of the cover crops as well as differential susceptibility of the cucurbit crops tested. Therefore, a plant-back period longer than the 8 days used in this study should be observed after cover crop incorporation before cucurbit seeding or transplanting.

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Erin C. Hill and Mathieu Ngouajio

Hairy vetch (Vicia villosa Roth) (HV) and cowpea [Vigna unguiculata (L.) Walp.] (CP) are two leguminous cover crops used in vegetable production systems. The residues of both species have been shown to suppress weeds via allelopathic interactions; however, they may also carry a risk of crop injury. A laboratory experiment was designed to study the dose response of carrot, sweet corn, cucumber, lettuce, onion, pepper, and tomato germination and radicle elongation to the aqueous extracts of both HV and CP. Aqueous extracts of fresh, whole plants were lyophilized to obtain a dry powder. Treatments of 0.00, 0.25, 0.50, 1.00, 2.00, 4.00, and 8.00 g dry extract/L of distilled water were applied to 10 seeds on filter paper in petri dishes. The petri dishes were then sealed and placed in the dark at 21 °C for 4 to 7 days, depending on the species germination. After the incubation period, germination rates and radicle lengths were recorded. Each treatment had 4 replications and the full experiment was executed twice. Pepper germination was reduced by increasing concentrations of HV extract; however, all other crops were not affected by HV or CP extracts. The HV extract had a significant effect on radicle elongation in carrot, corn, cucumber, lettuce, onion, and tomato. Inhibition of radical growth at 8 g·L-1 ranged from 42% in cucumber to as high as 81% in carrot. The CP extract had a negative effect on the radicle elongation of carrot, corn, lettuce, and tomato. Inhibition at 8 g·L-1 ranged from 42% in carrot to 67% in tomato. This study shows that both HV and CP extracts hold the potential to negatively affect the listed crops. Therefore, studies need to be done on the persistence of these effects in the field to maximize weed control while avoiding crop injury.

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Mathieu Ngouajio, William Kirk and Ronald Goldy

Nondestructive estimates of fruit volume are used for yield prediction. They are also used to study the relationship between fruit expansion rate and susceptibility to diseases or physiological disorders such as fruit cracking. A model relating bell pepper (Capsicum annuum) fruit diameter and length to its volume was derived using the equation of the volume of a sphere as the starting point. The model has the following formula: VF = KD2 Lπ/6, where VF is fruit volume, K is the shape factor that varies with fruit type, D is fruit diameter, and L is fruit length. The model is simple, easy to use in the field, and may account for variations in fruit shape. Regression analyses using actual fruit volume of bell pepper measured with the water displacement method and the volume estimated using different equations showed that accuracy of the new model is comparable to that of one of the best models previously proposed. However, because the model is less complex than previous models, it is easier to use in the field.