In tomatoes it can cause yellowing of leaves and shortening of stems, making them appear malnourished, except that the yellowing occurs in the tops of affected plants and the leaves may curl downwards. In potatoes, when symptoms show, the plant may be darker green than normal, the leaves are smaller with fluted margins, and the leaves near the ground are noticeably shorter and held upright. It is known only to occur in plants and poses no risk to human health. Dr Stephenson said the viroid has only been found in one tomato crop in Auckland, and the field survey will focus on checking major commercial tomato growers. Potato fields around the infected tomato crop will also be checked.
Laboratory tests are the only way to confirm the presence of the viroid.
The viroid can be eliminated by management practises, particularly through planting clean seed and crop rotation. It is spread principally from seed, and in tomatoes, between plants by the transfer of sap during routine management such as pruning. Aphids can also spread it, but only if the potato leafroll luteovirus, which is found in New Zealand, is present and acting as an encapsulating agent. MAF considers the occurrence of aphid transmission is unlikely at this point in time. The most likely source of entry of the viroid into New Zealand is by imported hybrid tomato seed, which is required by industry for their commercial crops.