The aim of this investigation was to determine the degree of susceptibility of various citrus varieties to die-back disease caused by Nattrassia mangiferae (H. & P. Sydow) Sutton & Dyko., with the use of excised twigs techniqe. Twenty five important varieties commercially grown in Khuzestan were tested and compared for their susceptibility to N. mangiferae. An appropriate spot in the middle of excised twig (30em long, 1-1.5cm in diam.) was first surface sterilized with 70% ethanol and then wound-inoculated with a 5-mm-diam. mycelium-agar-disk of highly virulent isolate of the fungus originally obtained from a diseased Lisbon lemon. Each uninoculated control twig received a Nattrassia- free agar disc. Wound was covered with a piece of parafilm and then wrapped with adhesive tape. Five replicate twigs of each treatment were incubated in a plastic container at 35± 1°C with R.H>90%. At the end of 7-days incubation period the bark was removed from around the inoculated spot, and the lesion boundary on the cambium was traced on transparancy, transferred to paper and it., size measured with a planimeter. The mean of lesion sizes were calculated and analyzed.Among nine lemon and lime varieties tested in the present study, Eureka, Meyer, Mexican lime and Lisbon, were highly susceptible with the average lesion sizes of 1863, 1840, 1704 and 1333 mm2 respectively, while Magdalina and Palestine sweet lime with averages of 715 and 225 mm2 showed least susceptibility. Of the five grapefruit varieties, Red blush and Ruby red with lesion sizes of 1813 and 1503 mm2 were highly susceptible, while Sham bar with an average lesion size of 877 mm2 showed the least suscetibility.Sweet and sour oranges and mandarins in general showed less susceptibility to N. mangiferae compared with limes, lemons and grapefruit varieties examined in this investigation. These results are further supported by the performance of the varieties in orchard, and with our experimental findings (wound inoculation of different varieties of citrus seedlings) in the nursery. Citrus varieties which possess high degree of resistance to N. mangiferae are urgently needed in Khuzestan province.

Mistletoe is known as a semi-parasite on fruit and forest tress in all of the world. This plant absorbs water and mineral nutrient of host and kill it gradually. In this study, many samples were collected from the fruit orchands and then distribution,precentage of damage and spesies of mistletoe were studied. The results showed. That infaction in the intermoutains areas, intercropping and lowerdensity orchard was more than others. The most contamination was on almond (Prunus amygdalus),apricot (Prunus armenica), pear (Pyrus communis) and very little in plum (Prunus domestica).This serui-parasiteplant causes dieback, loss in yeild and reduction in growth. The most loss of yeilds was in dryland farming almond (%59) and at least in plum (9%). The species of mistletoe in all of the hosts were Loranthus grewingkii Spach. The leaves of the species are thick, firm and in opposite pairs. The calor of the flowers is yellowish white and the kind of the fruit is berry. This species is reported for the first time from Iran.