Altered Middle Lobe Shape of Leaf Permits Rapid Diagnosis of Grapevine Fanleaf in Grapevine cv. St. George

Abstract

When potted plants of cv. St. George infected with grapevine fanleaf virus (GFV) were kept under the temperature conditions of 28/23C (day/night) or 20/17C, the newly developed leaves did not show typical symptoms of GFV such as wide petiole sinus, asymmetrical blade shape and leaf blades assuming the shape of a fan until 50 days after sprouting. The denticulation of the margins of the leaf blades on infected plants also showed almost no difference from those of healthy plants during the 50 days. However, the middle lobe cusps of these leaves on the infected plants were very acute after the above incubation period, and clearly different from those of healthy plants. The symptoms appeared faster under the higher temperature condition of 28/23C than under the lower temperature condition of 20/17C. Sharpening ratios for the cusps of the middle lobes of infected and healthy leaves were compared. Infected plants were indicated by ratios exceeding 2.6 in the 50 days period. On the other hand, healthy plants were indicated by ratios less than 2.6. A sharpening ratio of 2.6 was converted into a value of 42.4 degrees for the angle of the middle lobe cusp of a leaf. Grapevine fanleaf can be rapidly diagnosed in cv. St. George within a period of 50 days after sprouting on the basis of the following criteria. At temperatures of 28/23C, values below 42.4 degrees for the angle of the middle lobe cusps of three successive leaves from the fourth to the sixth in leaf order from the base of a shoot indicate infection with grapevine fanleaf virus. The measurement of this angle is easily made by using a magnifying glass with a V mark of 42.4 degrees, or with the naked eye.