An unidentified cause of yellowing in watercress was first reported by a farmer in September of 2000. After extensive efforts, e.g., laboratory analysis, pot and field tests, CTAHR’s virology laboratory identified a phytoplasma. The phytoplasma was confirmed to be the western North American aster yellows, or the severe aster yellows strain. Phytoplasmas are a group of microscopic organisms that cause over 700 diseases in plants. Phytoplasmas grow and multiply within host plants and insect vectors. In host plants, phytoplasmas are found only in the phloem tissue of leaves, stems, and roots. When the concentration of phytoplasma within the plant reaches a certain level, it is believed to cause hormonal imbalance, resulting in the development of certain symptoms. Symptoms such as chlorotic leaves, stunting, flower petals changing to a green color (phyllody or virescence), and witches broom (shoot proliferation) may appear.
HDOA confirmed a recently introduced leafhopper vector of the phytoplasma in October 2001. The leafhopper is known locally as the watercress leafhopper. This leafhopper has not been formally identified, but appears to be closely related to Macrosteles fascifons. This is the second phytoplasma to be identified in Hawai‘i; the first was on a native forest tree, the dodonaea.
After a noninfected leafhopper feeds on a phytoplasma-infected watercress plant, it takes about 2 to 4 weeks for the leafhopper to become a persistent vector. The infectious leafhopper feeds by inserting its mouthpart into the watercress phloem tissue and can then infect a noninfected plant. It may take several days, or longer, before symptoms such as chlorosis or shoot proliferation appear. However, noninfected leafhoppers may acquire the phytoplasma from infected watercress plants not yet showing symptoms. Also, the leafhopper lays it eggs into the leaves, petioles and stems of the watercress plant. Therefore, watercress from the Aiea-Waipahu production areas should not be used as planting material for other areas on O‘ahu or on neighbor islands.
Phytoplasmas can be spread via: 1) leafhoppers; 2) using infected planting material; 3) grafting; and, 4) parasitic plants (e.g., dodder). Phytoplasmas cannot be transmitted by rubbing sap from infected plants on to healthy plants or by cutting tools used in farming practices. Phytoplasmas are not known to be transmitted by seeds.