Antirrhinum Rust in New Zealand

New Zealand Journal of Agriculture, Volume 97, Issue 6, 15 December 1958, Page 551

Antirrhinum Rust in New Zealand

RUST disease of antirrhinums caused by Puccinia antirrhini Diet, and Holw. was first found in December 1953 on antirrhinum plants in Auckland and since then has been found in many other parts of New Zealand. Some details of work that has been done by the Horticulture Division of the Department of Agriculture on this problem are given in this article by R. Close, Scientific Officer, Crop Research Division, Department of Scientific and Industrial Research, Lincoln, and formerly Plant Pathologist, Department of Agriculture, Wellington.

GTHE disease was first recorded in California in 1895 and within 20 years it had spread over the whole of the United States. In 1931 it spread to France, and was recorded in England in 1933. It soon spread over most of Europe, and in 1940 it reached South Africa. More recently it has been recorded in Norway, Tanganyika,

Rhodesia, and Australia. The first record in Australia was made in October 1952 in a Sydney nursery, and within 18 months the disease had spread over most of Australia. The main host is Antirrhinum majus, a native of the Mediterranean region, where the rust was unknown. It seems fairly clear that the rust was indigenous on some Californian species of antirrhinum. That it has found the cultivated antirrhinum to be a very suitable host is shown by the fact that it attacks it at all stages of its growth and greatly mars its appearance and reduces its vitality. Effect of Disease All the above-ground parts of the plant except the petals are attacked and the first sign of infection is usually small yellowish spots on the leaves. These spots soon produce many brown pustules of spores, first on the lower surface and then on the upper surface. These rust pustules are produced on the stem, leaf stalks, flower stalks, sepals, and capsules. Those on the stem are usually elongate and may girdle the stem. . The pustules contain uredospores . (orange, summer spores) , but later dark teleutospores (black, winter spores) are produced. The fungus overwinters on old antirrhinum plants, which produce uredospores in spring and these infect young seedlings. There is no known alternate host. As with other rust fungi, various races of antirrhinum rust exist. Up

till 1937 only one race was known, but in that year the more virulent race 2 was found in America, and this was able to attack all the varieties that were resistant to race 1. It is possible that other races of this fungus occur. The causal fungus has been well described by Shirley D. Baker, of Plant Diseases Division, in the “Transactions of the Royal Society of New Zealand”, volume 83, page 453 (1956). Reports received from Horticulture Division officers have shown that the spread of the fungus in New' Zealand has been very rapid: December 1953: Auckland, first record. Autumn 1954: Hamilton, New Plymouth, Palmerston North, Levin, and Wellington. October 1954: Blenheim. December 1954: Christchurch. January 1955: Wairarapa and Nelson. •. February 1955: Gisborne and Hastings. November 1955: Roxburgh. January 1956: Dunedin. , . March 1956: Invercargill. April 1956: Gore. The rapid spread within New Zealand would be mainly by windborne spores and by the distribution of infected plants. In January 1955 plants of a Linaria species (most probably Linaria maroccana) were found in Wellington to be infected with antirrhinum rust.

This record is also extremely interesting, for only once before have linaria plants been found to be naturally infected with this rust. Conditions Favouring Spread Climatic conditions throughout New Zealand thus appear to be ideal for the growth and. spread of this particular rust. The optimum temperature for germination of uredospores and infection of antirrhinum plants is 50 to 60 degrees F., whereas the optimum for growth of the fungus is 70 to 75 degrees. Moisture is also necessary for infection to take place. It is fairly

evident that antirrhinum rust is a warm weather disease. Under conditions of high humidities and low night temperatures (50 to 55 degrees) combined with high day temperatures (70 to 75 degrees) the full cycle of the rust from infection to sporulation is completed about every 12 days. On the other hand very high day temperatures (90 degrees) and dry conditions retard the growth and development of this fungus. Secondary Fungi Injury to antirrhinum plants is, under moist conditions, mainly due to secondary fungi. These invade the rust pustules destroying the rust and advancing into previously healthy host

tissue, killing leaves and stems. In drier weather host injury results from the drying of rust-infected tissues. How Rust Could Have Entered New Zealand There seem to be two main theories on how this rust disease entered this country. The first is by seed transmission. Large numbers of both uredospores and teleutospores can be seed-borne. Viability of the spores depends on temperature and humidity. Seed-borne uredospores if stored at low temperatures and humidities will remain viable up to 4 months and could therefore be an easy means of rust dispersal. Teleutospores remain alive for some time, but like many other rust fungi they cannot reinfect the original host, in this case, antirrhinum. They do not therefore constitute a hazard. In the autumn of

1954 several consignments of antirrhinum seed from England and the United States were examined and all were found to contain both types of spores. These seed-borne spores could, if viable, affect seedlings in two ways: (a) Directly; that is, seedlings raised from infected seed could become infected by the spores borne on or with the original seed. (b) Indirectly; by the dusting of seedborne spores on to adjacent seedlings when the seed was being sown. The second way in which this rust could have entered this country is by wind from Australia. This theory seems to be a likely one because of the appearance of the rust more or less simultaneously in a number of localities throughout New Zealand. It is well known that dust can easily be blown across the Tasman Sea and it seems likely that the rust could have entered by this means. It is also possible that spores of the rust could have been carried on travellers’ clothing and thus could have entered from Australia in this way. Control Control of this rust disease can be attempted in three ways: 1. By good cultural conditions, thus preventing' the fungus from overwintering.

2. By the use of fungicides. In 1954-55 trials were carried out using the fungicides copper oxychloride, ferbam, and thiram. None of these materials proved satisfactory. Workers in several parts of the United States have shown that regular sprays with fungicides belonging to the ethylene bis dithiocarbamate group (dithane series) have given good control of the disease. Further research in New Zealand by Jacks and Webb (1955) confirmed this and also showed the value of a 75 per cent wettable sulphur as a control fungicide. To be effective, spraying must be carried out regularly at 10- to 14-day intervals. 3. By the growing of rust-resistant varieties. Resistance of plants to antirrhinum rust is controlled by a single dominant factor, so that a cross between a fully resistant plant and a susceptible one will yield 75 per cent of resistant plants and 25 per cent of susceptible ones. Modifying genes for resistance and the presence of two distinct races of rust make breeding work more difficult. In the 1954-55 season, plants of Wisley Bridesmaid were grown to identify the race of antirrhinum rust present in New Zealand, and these became infected. Since Wisley Bridesmaid is a race 1-resistant variety, this confirmed the suspicion that the rust was not race 1. Trials with Rust-resistant Varieties In the 1955 and 1956 seasons, trials with rust-resistant varieties were carried out by the Horticulture Division at both Auckland and Wellington.

Nine American varieties (Artistic, Campfire, Copper Shades, Loveliness, Pink Shades, Red Cross, Rosalie, Snow Giant, and Yellow Giant) were grown and these. showed varying degrees of resistance to this rust, though all of them became infected. Six varieties obtained from England became severely infected. These were Victory and Roselight and four of the strain ;1 -resistant varieties (Wisley Bridesmaid, Wisley Cheerful, Wisley Golden Fleece, and Watkin and Simpson’s Pink Freedom). Thus it is safe to assume that the antirrhinum rust in New Zealand is not race 1, but is similar to or identical with the American race 2. It is quite possible that both races are present. Three varieties of linaria, Linaria maroccana (Excelsior Mixed), Linaria Fairy Bouquet, and Linaria Sutton’s Special Mixture were raised and grown adjacent to antirrhinums, but none of these proved to be susceptible to antirrhinum rust. New rust-resistant varieties of antirrhinums will be obtained and tested in this country. Until such resistant varieties can be found antirrhinums, except under climatically favourable conditions, . will not regain their usefulness as bedding plants. References H. Jacks and A. J. Webb, “Glasshouse Tests for Control of Antirrhinum Rust”, “New Zealand Journal of Science and Technology”, vol. 38A, p. 309.