**11. Concluding remarks**

470 Soybean Physiology and Biochemistry

(Young & Hartwig, 1988). More studies are recommended. As Mundt (2002) demonstrated that for biodiversity to be functional, there must be an appropriate match between the resistant

Cover crops are commonly used to prevent soil erosion. These crops are usually planted in rotation with primary crops. When the cover crops are incorporated into the soil at the certain stage of the growing season, this practice is being referred as green manure. A major benefit obtained from green manures is the addition of organic matter to the soil, which increases the food supply for macro, and micro organisms in the soil resulting increased biodiversity in soil. There is a lot of information on the benefit effects of soil biodiversity on

This agriculture practice with certain crops which contain nematicidal compounds is especially interesting. Marigold, especially French marigold (*Tagetes patula*) has been shown reduced the populations in soil of several root-knot nematodes, and root lesion nematodes (Motsinger et al., Ploeg, 2000, Pudasaini, 2007). Castor beans, sesame, Sudan grass, sorghum, and Crucifers have all shown are toxic against plant parasitic nematodes. Among them, plants from Brassica have received considerable attention for their possibility in controlling plant parasitic nematodes by incorporating them into soil (Mojtahedi et al., 1993, Potter et al., 1998). The principle reason is that glucosinolates which exist in these plants convert upon decomposition to isothiocyanates, a group of chemicals proven to have a wide spectrum of biological activities, including nematicidal activity (Brown & Morra 1997), a few these chemicals are volatile, the practice has been referred "Biofumigation". Among these converted isothiocynates, allyl isothiocyanate (AITC) has been proven as being the most toxic against *H. glycines* (Lazzeri et al. 1993). AITC is the decomposition product of allyl glucosinolate (generally called sinigrin), which exists in plants of *Armoracia lapathifolia, Brassica carinata, B. juncea, B. napus, B. oleracea*, and *Peltaria alliacea* (Brown & Morra, 1997). Among them, mustards have been cited most promising, especially the oriental mustard (Brassica juncea) which contains highest concentration of Ally isothiocynate (AITC) in plant (Tsao et al., 2000). AITC toxicity was found highly selective, was highly toxic against J2 of *H. glycines*, but less toxic on

> 0 hr 0.5 hr 1 hr 1.5 hr 2 hr 2.5 hr 3 hr 3.5 hr 4 hr 4.5 hr **Time after mixed in soil**

Fig. 8. Effect of particle size of mustard materials on AITC releasing in soil

mesh 5-10 mesh 20-30

genes in a mixture and the virulence genes present in the target pathogens or parasites.

**10.3 Cover crop** 

disease control (Brussaard et al., 2007).

**AITC (ppm)**

The soybean cyst nematode is more likely going to spread to new soybean growing areas around the world as the climate change intensifies, and as the world becomes more integrated. The soybean seeds are more than ever developed and marketed by a few international companies, the soybean farming practices in the world will become more and more uniform, less diverse unfortunatly. New races could emerge. This creates greater challenges for managing the SCN.

The whole genome sequencing project of *H. glycines* has been completed by Monsanto Company and Divergence. The sequencing information although has been submitted to Genbank, it remains inaccessible to the public. DOE Join Genomic Institute led by Kris Lambert and Matthew E. Hudson (Univ. of Illinois at Urbana-Champaign) is in the process of sequencing the pest as well, in a hope that it will lead us to learn more about the races, and to find new ways for the controlling of the pest.

There are a few soybean germplasm collections in the world with the USDA collection being the largest, the holding information is accessible to the public. The Chinese soybean germplasm collection holds 6644 accessions of *Glycine soja*, a potential rich pool of source of resistance. Collaborations between the collections such as sharing information and germ lines are essential. The management of SCN must not rely on a few resistant cultivars. An integrated approach involving several cultivars, rotating with nonhosts, and cultivation practices that encourage biodiversity in the soil must be the future.

### **12. Acknowledgment**

Appreciation goes to Dr. W. Ye of Nematode Assay Section, Agronomic Division, North Carolina Department of Agriculture & Consumer Services, NC, USA for providing some of the pictures.

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