Interference Between Citrus Tristeza Virus (CTV) Isolates Detected by Analysis of Double Stranded RNA (dsRNA)

Interference between citrus tristeza closterovirus (CTV) isolates was studied by co-inoculating Madam Vinous sweet orange or Citron plants simultaneously or successively with mild and severe isolates, and monitoring plants for symptom expression and double stranded RNA (dsRNA) patterns. Twenty mild and two severe isolates were tested in several combinations. In most of the combinations assayed, three types of situations resulted: 1) the dsRNA pattern of the co-inoculated plants was the addition of the individual patterns, suggesting multiplication of both isolates without detectable interference; 2) the presence of the dsRNA pattern of only one of the isolates in the co-inoculated plant, suggesting exclusion or a drastic titer reduction of the other isolate; 3) new bands never before detected in plants infected by either isolate alone appeared in the co-inoculated plants accompanied by the pattern of one or both isolates. Plants inoculated only with mild isolates remained symptomless; whereas plants inoculated only with a severe isolate, and most of those co-inoculated with a mild and a severe isolate showed various degrees of symptoms. Whenever the dsRNA pattern of the severe isolate was detected in the coinoculated plants, alone or in combination with the pattern of the mild isolate or with new bands, symptoms of the severe type were observed. When this pattern was not detected, the plants were symptomless. This suggested that symptom expression requires detectable dsRNA levels of the severe strain and that analysis of dsRNA of the co-inoculated plants can be used for rapid screening of mild isolates for cross protection, so long as the dsRNA patterns of the mild and the severe isolate can be differentiated. The strong subgenomic bands detected in some isolates suggest the presence of defective RNAs in the infected plants