The Impatiens necrotic spot virus (abbreviated INSV) was first found and documented on California lettuce in 2006. Every year since then, INSV has occurred on lettuce to a greater or lesser degree throughout coastal California and especially in the Salinas Valley. Starting in June 2018 and continuing through July, we are seeing this year’s expected outbreaks of INSV in the Salinas Valley.
This virus can cause damage to all types of lettuce, including iceberg, romaine, leaf, and butterhead types. INSV-infected plants have leaves that overall can be yellow and have brown, necrotic (dead) spots within this chlorosis. The brown spotting can be observed on both older, outer leaves as well as younger, inner leaves. Severely affected leaves can be twisted or otherwise deformed. The brown necrotic tissue can resemble burn caused by pesticide or fertilizer damage. If plants are affected with INSV early in their development, growth may be stunted. INSV is in the tospovirus group of viruses and is vectored and spread by thrips insects. No other insect spreads INSV.
Management of INSV is difficult. Growers need to manage thrips as best as possible, which is a significant challenge. The virus can be present in weeds and other non-crop plants (especially ornamentals planted in home gardens), so field personnel should be aware of possible virus reservoirs in the vicinity of lettuce. Other vegetable crops are also hosts to INSV, such as radicchio, spinach, endive, and faba bean.
Pictured below are the various looks and symptoms of INSV on lettuce. The last photo is INSV on redleaf lettuce.