Transmission of tobacco necrosis virus by a fungus, Olpidium brassicae

Abstract

Experiments were performed to determine whether tobacco necrosis virus (TNV) entered plant roots at the same time and position as zoospores of Olpidium brassicae (Wor.) Dang. Mung bean ( Phaseolus mungo L.) seedlings were immersed in a combined suspension of Olpidium zoospores and TNV and heated 10 seconds at 50° to kill the Olpidium without inactivating the virus. TNV infection did not usually occur if the roots were heated within 2 hours, but did occur if the roots were heated at 3 or more hours after immersion. This lag of 2 to 3 hours before TNV infection corresponds with the time it takes for Olpidium to penetrate roots. However, when mung bean seedlings were placed in a suspension of Olpidium zoospores for 3 hours, heated to destroy Olpidium, and then placed in a TNV suspension, no infection occurred. Thus, the Olpidium did not appear to predispose the roots to subsequent TNV infection. Olpidium zoospores encysted on and entered the roots of 4-week-old lettuce ( Lactuca sativa L.) plants mainly within 5 mm of the root apex, and subsequent assays showed that the apical 5 mm of the root was also the zone where most TNV infection occurred. TNV infection of roots was induced only when actively swimming Olpidium zoospores were able to encyst on roots in the presence of TNV. Mild treatments, such as aging for 1 2 hour or more, heating to 35° for 10 minutes, and addition of 10 −3% CuSO 4. 5H 2O permanently inactivated the delicate zoospores, and prevented TNV infection of plants subsequently placed in the suspensions. Washings of Olpidium-infected lettuce roots, if taken during the first 2 minutes of washing, did not contain zoospores and did not induce TNV infection of mung bean seedlings. However, washings taken after 2 minutes did contain zoospores and did induce TNV infection. The amount of TNV infection was approximately proportional to the number of zoospores. The above is considered strong evidence that Olpidium zoospores transmit TNV in a vectorlike manner. In line with results with Californian strains of TNV, Olpidium zoospores transmitted a British strain of TNV to cowpea [ Vigna sinensis (Torner) Savi], mung bean, tomato ( Lycopersicon esculentum Mill.), and tobacco ( Nicotiana tabacum L.) roots. Thus, it appears likely that Olpidium transmits TNV in areas other than California.