Sudden oak death (Phytophthora ramorum) was discovered in a small number of rhododendron bushes along one of the park’s trails.
Visitors have been told to stick to pathways and keep dogs on a lead in signposted areas in order to prevent spreading.
The Forestry Commission said there was no risk to human health.
Mike Seddon, deputy surveyor for the New Forest, said: “As a result of routine monitoring undertaken as part of national measures to protect Britain’s trees and forests against Phytophthora ramorum, infection has been confirmed on some Rhododendron ponticum bushes.
“We are now working with Defra to determine the exact scale and extent of the outbreak, and to destroy infected plants,” he added
“In the meantime the public may continue to enjoy visiting the New Forest as usual.
“However, to help prevent the spread of the disease, we ask that in signposted infected areas they stay on the footpaths, keep dogs on leads, and do not take plant cuttings. There is no risk to human health.”
Sudden oak death was first identified in California, where it has made tan oak trees a rarity.
It causes roots and leaf discolouration, resulting in the death of the infected plants.
In the UK, rhododendron and viburnum are most commonly affected.
Mr Seddon added that, despite its nickname, sudden oak death actually poses little risk to the New Forest’s oak trees.
“There is, however, evidence that other species, such as beech and ash, are susceptible,” he said.
“Our approach therefore is to find out exactly the extent and severity of the outbreak and destroy the infected shrubs and plants to minimise the risk of the infection spreading into the New Forest’s trees.
“We have set up a survey of the area within 3.0km (1.8-mile) of the outbreak.”
Signs have been erected in the forest giving more information.
Source: BBC News website