Botrytis porri Buchw . on leek as an important storage fungus in Finland

Botrytis porri Buchw. was found to be a common and economically significant pathogenic fungus on leeks in storage. The increase in the number of fungi caused a linear decrease in the number of marketable leeks when stored at 0.5° C. B. porri was found to spoil leeks even at —o.s° C. Spraying with benorayl and thiophanatemethyl one or two weeks before harvesting significantly decreased the numbers of the fungus and the amount of damage caused during storage. Botrytis allii Munn and Fusarium avenaceum Sacc. rarely caused spoilage of leeks.


Introduction
Leeks come second to onions as important Allium plants in Finland. The area under cultivation has increased from about 30 ha to 80 ha during the 1970'5. The value of the crop is high since the price of leeks is about three times that of onions in Finland. Leeks are stored in the field in more southerly countries, but owing to the severe winter in Finland they have to be moved to temporary stores or refrigerated stores. Although rather extensive studies have been carried out in Finland (Suhonen 1970) on storage techniques and conditions, diseases occurring during storage have not been studied at all. In practice, however, the most serious risk factor in the storage of leeks has proved to be storage diseases. Rather little research has been carried out abroad on the storage diseases of leek presumably as a result of the specialised cultivation technique required. In Norway, the most important fungal pathogen of leeks in storage has been found to be Botrytis porri Buchw. (Roed 1952). Pathogens are considered to be a serious problem in the storage of leeks in Norway ( Hoetun 1978 a).
The aim of this study was to determine the fungal pathogens of leeks in storage, their significance and possible ways of controlling them by means of fungicide sprays before harvesting.

Material and methods
Storage experiments were carried out in 1975/76, 1976/77, 1977/78 and 1978/79. The stored leeks, variety »Copenhagen marget», were grown each year on the same site in Viikki which had not earlier been used for growing leeks. The seedlings were grown in the greenhouse, planted during the period 15.-25. 5. and the crop harvested during the first or second week in October. The field, which was of good fertility, was given a basic fertilization each year of 1 200-1 400 kg/ha of chloride-free compound fertilizer (N : P a 0 6 : K 2 »J = 7:24 : 14) and nitrogen was added 2-3 times during the growing season as calcium nitrate to give a total annual nitrogen dosage of 250 kg N/ha. The leeks were placed in a refrigerated store at 0-(-1°C and a relative humidity of 96-98 %. From 10 to 15 normal-sized leeks per replication were placed in open, perforated (0 1 cm, at intervals of 15 x 15 cm) plastic bags.
Before storage, the roots of the leeks were trimmed to a length of about 1 cm and the leaves shortened by about 1/3. At the end of the storage period, the fungi on each leek were determined and the degree of fungal infection estimated using the scale 0-5. The leeks were then prepared for marketing in the normal way. The fungus determinations were carried out using a stereomicroscope. Whenever necessary, plant samples were placed on moist filter paper in petri dishes for more precise determinations and for preparing pure cultures.
Spraying was carried out on the field one or two weeks before harvesting using different types of fungicide. The growth of Botrytis porri at different temperatures was studied in 1977/78.
A piece of B. porri agar was placed on the upper part of a leek which had been cut to a length of about 10 cm. The leeks were stored on damp filter paper in containers covered by plastic film. Eight infected leeks were incubated at each temperature: -0.5°± 0.5°, +l°± 0.5°, +4°± 0.5°and +B°± 1°C.
A similar experiment using leek discs was carried out the previous year. The results of this test will not be discussed here as they were in complete agreement with those obtained in 1977/78.
Leek samples obtained during the course of the experiments from leek stores situated in different parts of Finland were studied in order to determine the types of fungi which affect leeks in storage.

Botrytis porri Buchw. on stored leeks
Botrytis porri (syn.: Botryotinia porri (van Beyma) Whetz., Sclerotinia porri van Beyma) produced 9-ll pt (11.7 pi) thick, initially light coloured, later grey mycelia. Conidiophores and conidia (Fig. 1) were formed on the host    plants, but conidia were not formed at all on potato dextrose (PDA, Difco) agar. The conidia were 11 -lB pi (14.5 pi) x 9-12 pi (10.8 pi) in size. The fungus produced 3-lo mm, irregular-shaped sclerotia on the PDA medium (Fig. 2) and also on leeks (Fig. 3). The sclerotia were initially light grey in colour, but as they aged turned almost completely black. When leeks were stored in the normal way, the plants lying closely packed together, sclerotia were rarely formed. The fungus produced large numbers of sclerotia in the autumn in the field and in the temperature experiments where the leeks were kept separate from each other. The fungus was found in all parts of the leek plants. B. porri grew extensively throughout the plant tissues and also grew out between the leaves. The infected plant tissue became soft and watery (Fig. 4). Leeks which were packed tightly together became very slimy and appeared at first glance to have been spoilt by bacteria.
B. porri was found in all the areas studied: the Helsinki, Turku and Jyväskylä areas and the Aland Islands. On the basis of discussions carried out with farmers and consultants, it is apparent that the fungus is also to be found in other areas where leeks are cultivated. B. porri was usually found only on leeks which had been stored, but in 1977 and 1978 the disease was already apparent in October on the upper parts of leek plants growing in the field. The fungus did not cause any damage in the field.
At Viikki, where the disease was followed for four years in succession, the fungus occurred in less than 1 % of the leeks during the first year, in about 10 % ot the leeks during the second, and in 80-90 % of the leeks in the next two years. The increase in the amount of B. porri in stored leeks brought about a linear (y --0.56 x 73.8) decrease in the marketable proportion of leeks at 0.5°C (Fig. 5). After four months storage, a B. porri content of 80 -9O % produced almost complete spoilage. B. porri was still able to grow on the leeks at a temperature of -o,s°C and caused a serious amount of spoilage in 4 months (Fig. 6). Raising the temperature to 8°C speeded up spoilage.

Control of Botrytis porri
Spraying with benomyl and thiophanate methyl before harvesting significantly reduced the B. porri content and also the storage losses (Tables 1 and 2). In 1977/78, when the fungus was artificially spread on the field, two sprayings gave better results than one only. In the last year of the experiment, spraying carried out two weeks before harvesting was as effective as two sprayings, but better than spraying carried out one week before harvesting. The same result was obtained with both preparations. The best treatment reduced the fungus content from about 90 % to about 20 %.
Non-systemic control chemicals did not reduce the B. porri content of the stored leeks (Table 1).
At harvesting, the amount of fungicide residues present in leeks sprayed two weeks before harvesting with benomyl and thiophanate methyl was always less than 0.4 ppm. No residues were found in samples taken at the end of the storage period.  Other fungi on stored leeks Bolrytis allii Munn was rarely found in the stored leeks. This fungus causes spoilage of leeks in a similar way to B. porri, but owing to its rare occurrence it was of no importance as a storage fungus.
Fusarium avenaceum Sacc. was found on a very few occasions in spoiled leeks. Spoilage always started from the lower part of infected leeks. Infected tissue was red-brown in colour.

Discussion
In this study, grey mold of leek has been called Bolrytis porri Buchw., although in some studies (Cronshey 1947, Röed 1952, Cook 1976) the conidial stage of Botryotinia porri (Van Beyma) Whetz. is considered to be identical to Bolrytis hyssoidea Walker. B. porri forms large sclerotia on PDA medium (cf. Röed 1952) but B. hyssoidea does not form sclerotia at all on PDA medium (Walker 1925). Buchwald (1949), Ellis (1971) and Jarvis (1977) consider the conidial stage of Botryotinia porri to be different from B. hyssoidea. B. porri was now found for the first time in Finland. As the fungus is an extremely common and damaging storage pathogen of leeks, it has undoubtedly been present in Finland for a long time. It has possibly been confused with B. cinerea Pers., as has occurred elsewhere (Röed 1952). It may have been introduced to Finland along with imported seed as it is also a seed-borne pathogen (Neergaard 1945). According to the observations made in this study, infection of the crop in the field takes place during the end of summer and autumn before harvesting. The fungus may be already visible, but in most cases it is hidden (cf. Hoftun 1978 b). Continuous cultivation of leeks appears to strongly increase the incidence of the disease.
B. porri can grow at temperatures below 0°C. According to Hoftun (1978 b), it can even take place below -2°C. Control of the disease by lowering the storage temperature is therefore not possible. Adding carbon dioxide to the air in the store reduces or inhibits the growth of B. porri (Hoftun 1978 b), but the construction of storage space in Finland where the air mixture can be controlled, has not become common obviously as a result of the high costs.
Control of B. porri should therefore be done in the field before harvesting.