Tulip Breaking Virus - Viral Strains

Viral Strains

Two separate strains of the virus – Severe Tulip Breaking Virus (STBV) and Mild Tulip Breaking Virus (MTBV) – have been determined by the type and severity of the symptoms they cause. In certain varieties STBV causes full breaking or light breaking, when due to a lack of anthocyanin, pigment present in the surface chromoplast cells fades and the lighter color (white or yellow) of the internal mesophyll is exposed, appearing as irregular streaks or fine featherings. In the same varieties MTBV causes self breaking or dark breaking, which occurs because anthocyanins are formed in excess, so the color in the epidermal cells is intensified in dark streaks or elongated flecks and sworls. Breaking commonly occurs along the edges and tops of petals and sepals. The most common type of breaking found in naturally infected plants, called average break, is caused by infection with a mixture of STBV and MTBV; both light breaking and dark breaking symptoms are present, together with some unbroken areas, in different parts of the same petal. Plants infected with a mixture of strains when young may exhibit both effects, with light breaking symptoms restricted to the basal part of the petal and severe dark breaking effects in the upper parts. Some varieties were later found by to be incapable of light breaking, and always showed dark breaking symptoms no matter whether infected with STBV or MTBV or a mixture of both strains. Although they still can be infected, white and yellow flowered varieties are incapable of breaking since they lack anthocyanins; their color is determined by colourless or yellow plastids in the mesophyll. Other outward symptoms can include leaf mottling.