TOMATO - VIRUSES

Host range: IYSV has a relatively restricted host range. Edible Allium crops including onion (bulb and seed crops), garlic, chive, shallots, leeks and some cut flower/potted ornamental species including Alstroemeria, chrysanthemum, iris and lisianthus are the most economically important crops affected by IYSV. Wild Allium species and ornamental alliums are also potentially at risk. A range of weed species (Datura stramonium, Nicotiana spp. and Amaranthus retroflexus) can also act as reservoirs.

Occurrence and importance: Iris yellow spot represents an immediate and serious threat to sustainable and productive onion cropping systems around the world, and the recent detection of this disease in numerous onion-producing countries demonstrates that the disease is spreading rapidly in a range of environments. This virus is spread by seeds and it is a quarantine pest in many European countries. Farmers must be able to identify this virus because the continuously changing climate conditions and global seed markers contribute to its wide distribution. The early identification helps farmers to minimise the damages in crops. The climate change improves its occurrence because of the spread of  Thrips ss.

Symptoms of IYSV are eyespot to diamond-shaped, yellow, light green, or straw-colored (sometimes necrotic) lesions on leaves, petioles, and bulbous leaves of onions and other Allium host species. In the early stages of infection, the lesions appear as oval, concentric rings. Some green islands are observed within the necrotic lesions. They usually come from the feeding site of thrips. Infected leaves eventually fall off at the site of infection in the second half of the growing season. The infection causes yield loss in the early stages of plant growth. In the later stages of development, infection can still cause significant losses due to quality degradation: heavily infected areas become prematurely aged and whole areas turn brown before collapsing. The severity of symptoms depends on the host species, the time of infection, the general health of the host at the time of infection, and environmental conditions.

With a few exceptions, all onion varieties show a significant reduction in yield and a reduction in bulb size. The virus reduces plant vigor, disrupts photosynthesis and reduces bulb size. IYSV infection weakens plants, making them more susceptible to other diseases and pests. Seed yield and quality of bulbs grown for seed infected with IYSV are reduced.

Transmission and Epidemiology: IYSV is a vectored virus, so two organisms are involved in the initiation and spread of the disease. This datasheet focuses on the virus. Tospoviruses are usually transmitted by a large number of thrips species; however, IYSV is only transmitted by onion thrips (Thrips tabaci). IYSV is transmitted by both larvae and adult thrips, but only the larvae can acquire the virus from infected plants. Virus transmission is persistent and once a thrips has acquired the virus, it can transmit it for the remainder of its lifetime. IYSV is likely to overwinter from one season to the next in volunteer onions or weeds found among or around crops. Emerging thrips spread the virus from infected to healthy hosts whilst feeding. The disease has the potential to spread rapidly in fields with large numbers of viruliferous thrips. The distribution of infected plants in the field is associated with feeding activity by the vector. In many cases, the damage is first noticed at the field edges, in areas of stressed plants, or in locations with thin plant stands. The virus is not seedborne nor does it survive in the soil.

Control: Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Control measures for tospoviruses need to be deployed within integrated pest management strategies which include phytosanitary, cultural, host-plant resistance, chemical and biological measures. Volunteer onions plants and weeds acting as reservoirs should be destroyed (either by tillage or herbicide) and weeds should be actively managed around cropping systems and within fields. Crop rotation should be implemented to reduce the build-up of thrips populations and transplants free of IYSV and thrips should be used. Studies have shown that thrips are less attracted to green hues than blue hues; green-leaved onion cultivars are less attractive to thrips and therefore less likely to be infected with IYSV.