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<strong>Scholarly</strong> J. Agric. Sci. 134 Figure1: The symptomes of white tip nematode (Aphelenchoides besseyi) Figure 2: Rice cluster with hard infestation to white tip nematode (Aphelenchoides besseyi ) in rice field of the western part of the Mazandaran but about 8/7% samples didn’t show the infestation. Figure 3 shows the infested samples number which harvested from west regions of the Mazandaran. As infers from this Figure, infested samples percent of the western regions of the Mazandaran to A. besseyi nematode are: 81/7% Ramsar, 92/3% Tonekaboon and 100% chaloos and infestation general average of the sampling regions is 91/3% of 100 seed samples. Disease Symptoms Nematode symptoms in rice leaves are in form of the tip of the leaves become white and to turn about 3-5 cm, finally it will become brown and break (figure1). Delay in appear of cluster, decrease of seeds and plant growth,
Mahdavian and Javadi 135 120 100 80 Ramsar 60 40 20 0 Number of sample Number of sample infection Percentage of infection Tonekabon Chalos Figure 3. Percent contamination of rice seed samples in different regien of West parts of Mazandaran late crops and produce tiller from upper nodes are the symptoms of the nematode (figure 2). Disease Factor Aphelenchoides besseyi nematode is a factor of white tip of the rice leaf. This nematode belongs to Aphelenchoididae family. DISCUSSION Be extended of A. besseyi distribution regions in rice fields of the west Mazandaran are due to infested rice seeds. White tip nematode was seed-borne and it outspreads by infested seed Ou (1972). High degrees of infested seeds in thesaurus bed facilities the extent of the regional infestation Luc and Sikora (1990). Infestation from soil occurs seldom. Just 5 plants of 2900 plants which produced from healthy seed grow in infested soil showed disease symptoms, Huang (1959). Infestation percent will increase when healthy and unhealthy seeds grow beside each other in a same field. Irrigation water which use in thesaurus bed contains nematode and ill membranes, also it cause to facilitate healthy seeds infestation. All achieved experiments in Japan show that nematode transfer in field result from germinated seeds or transplanted seedling Yoshii and Yamamoto (1950). According to study the source of the A. besseyi economic damage threshold including association density determine in specific number of seed or seed weight. Fukano (1962) determined the economic damage threshold of this nematode was 300 fresh nematodes into 100 seeds, although various researchers mentioned differently, 100 nematodes in 100 seeds or 2-400 nematodes in 100 seeds. Quality number in 1gr seed (paddy rice) is intermediately 40-45. According to present study, 20 regions showed the infestation higher than 500 nematodes in 10 gr seed (intermediately, 1/7 nematodes in each seed). So rice crop damage can be expected by this infestation amount in above regions. As pointed before, nematode association in 10 gr seed was more than 1000 nematodes in 5 regions that is there’s intermediately 3/6 nematodes in each seeds which according to economic damage threshold gave by Fukano (1962) can be injurious. In this study, extracted nematode association of seeds evaluated less than 100 nematodes in each 10 gr seed in 21 of 100 regions because probably this amount of nematode association didn’t have crop damage for farmers. Although some researchers mentioned 2-400 nematodes in 100 seeds in economic damage threshold. On the other hand, all patient plants don’t show the symptoms. Infected plants may have no symptoms and even decrease can be see when seeds contain nematode, the characteristic symptoms of the disease don’t occur. During a study in Japan, about 75% Infected plants didn’t show symptoms Yoshii and Yamamoto, (1950). If also pollution amount was 0.7 %, the economic value of polluted seed will confront with decrease Inagaki (1985). Study results show that more resistant plants indicate seldom symptoms in clusters and leaf, whereas they contained nematode and also their crops decrease Atkins and Todd, (1959). More than 50% rice field which infested to this nematode was reported in Bangladesh. Clusters weight that were more infested (650 nematodes in 100 seeds) have been a third as compared with plants which had less infection (112 nematodes in 100 seeds) Rahman and Taylor, (1983). Nematode association tolerance estimated 30 live nematodes in 100seeds. This number cause to 50% damage to crop Fukano (1962).
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