Studies on some viruses infecting onion,Allium cepa L.,plants in Egypt

Two viruses used in this study, Onion yellow dwarf virus (OYDV) and ýTobacco rattle virus (TRV), were isolated from naturally infected onion ýý(Allium cepa L.) plants growing in the fields of onion plants during the ýsurvey carried out in 2007-2008 and 2008-2009 in seven Egyptian ýGovernorates. The results indicated that the mean percentages of ýOYDV-infection were 24.48 and 19.98 % and TRV-infection were 1.04 ýand 0.42%, respectively in first and second seasons all over the seven ýGovernorates. Naturally infected onion plants showing symptoms of ýOYDV i.e. leaf curling, stunting, malformation and yellow striping and ýTRV i.e. yellowing, malformation, chlorotic and white necrotic stripes. ýThe obtained results indicated the possibility of mechanical ýtransmission of the two viruses, which were also transmitted by seeds ýwith percentages ranged between12-18% for OYDV and 8-13 % for ýTRV. Meloidogyne incognita nematodes were able to transmit TRV to ýhealthy onion plants by 12%. The host range of the two viruses included ýý7 different plant families. Cytological study using light microscopy ýresulted the formation of amorphous inclusion bodies in the cytoplasm ýof infected onion plant Beheri leaves with OYDV and tobacco leaves ýinfected with TRV. By light microscopy of semi thin sections of both ýhealthy and artificially infected onion leaves with the two viruses several ýanatomical changes were observed reflecting the external symptoms on ýinfected plants. By electron microscopy, the two virus particles were ýobserved with two main dimensions (length 770 nm and 12nm width for ýOYDV and tubular particles with straight i. e. long about 190 nm, short ýabout 114 nm with diameter about 22 for TRV). Investigation of ýultrathin sections by Transmission electron microscopy revealed ýchanges in the chloroplast and in the cytoplasm where showed ýpinwheels and scrolls with OYDV only, in both the nucleus and the ýchloroplast with TRV. The different serological methods were used ýsuccessfully for detection of the two viruses i.e. DAS-ELISA, indirect ýELISA, TBIA and DIBA. According to the available data, TRV was ýisolated and identified for the first time in Egypt from onion plants ýduring the present study. Changes in some biochemical components ýassociated with OYDV infection were studied, using two onion cultivars ýGiza 6 and Beheri. The virus OYDV reduced the photosynthetic ýpigments (chlorophyll A, B and total chlorophyll) and carotene contents ýof infected onion leaves Beheri. In addition, percentages of reducing, ýnon-reducing and total sugars were increased in bulbs of both cultivars, ýbut reduced in leaves of Beheri. The results indicated that the ýpercentages of free, conjugate and total phenols were reduced in bulbs ýof both cultivars, while increased in leaves of Beheri in infected onion ýplants. Concerning peroxidase activity was increased in infected bulbs ýof Giza 6 followed by leaves and bulbs of Beheri, but polyphenoloxidase ýactivity was increased in the bulbs of Beheri, Giza 6 and leaves of ýBeheri.ý