Root Gall Index

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Nematode Population

In experiment A, all treatments significantly reduced nematode population in comparison with the untreated fallow (76.9%). The highest percent reduction in nematode population was observed in the Methyl Bromide treatment (99.7%) followed by the plots planted with oil radish covered with plastic (98%), with no significant difference between these two treatments. However, nematode population reduction in both treatments (Methyl Bromide or oil radish covered with plastic) was significantly higher than the plots planted with oil radish without plastic cover (89.6%).
In experiment B, the highest reduction was once again achieved with Methyl Bromide (99.7%). In plastic covered arugula or oil radish plots, M. incognita population was decreased by 97.7% and 94.2%, respectively, whereas in uncovered oil radish plots, population reduction was only 86.3%. In the untreated fallow, nematode population reduction was as low as 74.7%.
In both experiments A and B, all treatments significantly reduced nematode population in comparison with the untreated fallow. Furthermore, there was no significant difference between the percent reduction of nematodes achieved by any of the two green manure crops (oil radish and arugula) covered with plastic and Methyl Bromide. When covered with 50 microns transparent polyethylene film immediately following their incorporation to the soil, both oil radish and arugula have been as effective as Methyl Bromide in reducing nematode population in the soil.
Generally, in both experiments A and B both biofumigation crops used significantly reduced nematode population as compared to untreated fallow (control). This result is conform to findings of other studies describing oil radish and arugula as non-hosts (or least poor hosts among a variety of biofumigant crops tested) to root-knot nematodes, causing decline of soil populations by starvation at the first place (Curto et al., 2005; Liebanas & Castillo, 2004; Pattison et al., 2006). Results of our experiments also match other studies having found that these biofumigation crops are able to control nematodes (Buskov et al., 2002; Mojtahedi et al., 1991; McFadden et al., 1992; Spak et al., 1993; Angus et al., 1994), and more specifically those having associated the nematostatic effect of biofumigation crops to the release of glucosinolates which soon convert into isothiocyanates in the soil and thus suppress nematode populations (Morra and Kirkegaard, 2002).
In both experiments A and B, plastic cover has significantly increased the efficiency of oil radish and lead to better nematode control. These results indicate that plastic cover enhances the effectiveness of the biofumigation crops used in terms of nematode control. It is to note that there was remarkable difference in the initial soil populations of nematodes in plots treated with covered and uncovered oil radish in each of these two experiments. In fact, the average initial infestation with root-knot nematodes in all eight plots treated with oil radish (covered and uncovered) at the site selected for experiment A was considerably more severe than in plots treated similarly at the site where experiment B was conducted, as shown in table # 1.1 below. Furthermore, whereas in experiment B the average initial infestation in plots treated with oil radish and covered with plastic was lower (865) than in plots treated with oil radish and kept without cover (3,475), in experiment A plots treated with oil radish and covered with plastic had considerably higher average initial infestation (12,430) than those of with oil radish without cover (7,310).

Temperature Effect

Soil temperature records registered at 30 cms depth in the four replicated areas treated with oil radish and covered with plastic varied between 20 and 28 oC over the entire period when soil was covered with plastic (from 18 September until 2 October). The highest temperature reached was 28 oC (replicate 2). Table # 1.2 below summarizes the values recorded. It is to note, that the highest temperature reached in each of the four replicated areas persisted for either 24 hours (replicates 2 and 3 – 28 and 27 oC respectively) or a maximum of two days (replicates 1 and 4 – 27 and 25 oC respectively).Through a laboratory experiment on the effect of temperature on the root-knot nematode M. incognita, Wang & McSorley have determined that while 100% of J2 were killed when exposed to a temperature of 42 oC for only 13.8 hours, no mortality was recorded at temperatures below 38 oC, no matter how long was the exposure period extended. Furthermore, the minimum temperature found effective on the J2 population was 39 oC, but a period of 47.9 hours was needed to kill 100% of J2 at this temperature (Wang & McSorley, 2008).
In another field experiment, Lamberti et al. have found that temperatures of 35 oC and 45 oC had little effect on M. incognita and did not increase lettuce yields in Italy (Lamberti et al. 2000). Thorough literature review reveals that research on the correlation of the temperature effect with biofumigation is not abundant. However, most of the few available studies indicate that soil amendment with biofumigation crops does not have any suppressive effect on root-knot nematodes below soil temperatures of 20 oC (Ploeg and Stapleton, 2001; Bello et al., 2004; Lopez-Perez et al., 2005).
On the other hand, many studies have associated temperature with root-knot nematode control through soil solarization. Most of these studies indicate figures varying between 35 and 38 oC as lethal temperature for Meloidogyne incognita.
Results of our experiments indicate that use of plastic mulch to cover soil treated with oil radish has always increased the effectiveness of this biofumigation crop and lead to a better control of the root-knot nematode. Findings of the experiments mentioned above (Wang & McSorley, 2008; Lamberti et al. 2000) and soil temperature levels recorded in experiment A, lead us to conclude that the additional effect of plastic cover on the root-knot nematode in this experiment was not due to the increase of soil temperature (since temperature levels achieved were far from the lethal temperature levels), rather than the capacity of the plastic in maintaining the isothiocyanates released by the oil radish crop in the soil for longer periods. Without the presence of these isothiocyanates, even the highest temperature recorded at any of the four replicates would not be sufficient alone to suppress the root-knot nematode population. This assumption is also concomitant with the outcome of the research conducted by Stapleton and Duncan, who found that covering the soil treated with residues of broccoli and other cruciferous plants can control M. incognita even at sublethal temperatures (Stapleton and Duncan, 1998), as well as with the findings of Morra and Kirkegaard, who have associated the nematostatic effect of biofumigation crops to the release of high concentrations of glucosinolates, which soon convert into isothiocyanates in the soil and thus suppress nematode populations (Morra and Kirkegaard, 2002).
Our results also indicate that oil radish variety “Boss” and arugula fit the definition of Viaene and Abawi (1998) of a good cover crop due to the suppressive effect they showed on the nematode population in the soil when applied as green manure.
Finally, the above findings confirm that both oil radish variety “Boss” and arugula can suppress root-knot nematode populations in greenhouse production of cucumbers in Lebanon, acting first as a poor host by preventing buildup of root-knot nematode populations, as well as through their characteristics of a biofumigant green manure crops producing glucosinolates (biocides) when incorporated to the soil.

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Root Gall Index

In experiment A root galling was significantly reduced by application of all treatments compared to fallow. The lowest root gall index was observed in the plots treated with Methyl Bromide (0.9). There was a significant reduction in the oil radish planted plots either covered with plastic or left uncovered compared to fallow. The gall reduction was less in plots planted with oil radish with or without plastic cover when compared to the untreated fallow plots (1.7 and 2.0 respectively). The highest galling was found in the roots of untreated fallow plots (7.1).
Both experiments A & B revealed similar results. Both biofumigation crops used significantly decreased root galls as compared to the untreated control. These results confirm the findings of several earlier studies classifying arugula and oil radish as non-host or poor host species to the root-knot nematode (Melakeberhan et al., 2006; Curto et al., 2005; Pattison et al. 2006). The significant decline of root galls under the influence of these biofumigation crops justifies also the reduction of the soil population of the root-knot nematodes discussed earlier. Obviously, when there are no juveniles (J2) or active juveniles proliferating in the soil, the root gall formation on the roots of the crop will definitely reduce.
These results further confirm that oil radish variety “Boss” and arugula can be considered as good cover crops and cause decline of the root-knot nematode population in the soil and on the roots of greenhouse cucumbers under Lebanese conditions.

Fruit Yield

In experiment A where the total yield was based on the harvest of 10 plants per plot, the highest cucumber yield was obtained from plots treated with Methyl Bromide (11,054 grams), followed by oil radish covered with plastic (8,780 grams). Oil radish without cover yielded 7,165 grams, while yield resulting from untreated plots was as low as 4,940 grams.
In this experiment, all treatments increased cucumber production significantly as compared to the untreated fallow (control).

Table of contents :

CHAPTER I
INTRODUCTION
MATERIALS AND METHODS
 Experiment A
 Experimental design (Experiment A)
 Experiment B
 Experimental design (Experiment B)
RESULTS AND DISCUSSION
 Nematode Population
 Temperature Effect
 Root Gall Index
 Fruit Yield
LITERATURE CITED
CHAPTER II
INTRODUCTION
MATERIALS AND METHODS
 Experimental design (Experiment C)
RESULTS AND DISCUSSION
 Nematode Population
 Root Gall Index
 Fruit Yield
LITERATURE CITED
CHAPTER III
INTRODUCTION
MATERIALS AND METHODS
RESULTS AND DISCUSSION
 Experiment A (Chapter I)
 Experiment B (Chapter I)
 Experiment C (Chapter II)
LITERATURE CITED

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