Celery and parsley were originally cultivated for medicinal purpose, and it was later developed into a food and flavoring crop. It is attacked by a number of diseases that affect yield and quality. Septoria leaf spot is an important foliar disease of both celery and parsley crops worldwide.

The aim of the study was to investigate in more detail the pathogen Septoria apiicola and S. petroselini, occurring very often in Yugoslavia.

Many strains of a pathogenic fungus were isolated from diseased celery and parsley leaf tissue. Leaf spot symptoms, characteristics for natural infection, were also reproduced by artificial inoculation of plants in greenhouse. Influence of temperatures and leaf wetness on disease severity was investigated. Pathogen survival in the leaf debris and on the seed were studied. The objective of the research was pathogenicity of isolates of S. apiicola and S. petroselini on different host plants and susceptibility of celery cultivars to S. apiicola.

Leaf spot or blight, caused by Septoria apiicola (Speg.) and S. petroselini (Lib.)Desm. is the most frequent and destructive foliar disease of celery and parsley in the world (Sherf and MacNab, 1986; Lacy et al., 1996). S. apiicola and S. petroselini are the most important economic pathogens in celery and parsley in Yugoslavia. The disease is usually present in growing regions each year. In favorable conditions, rainfall and wetness periods, disease spread is very rapid and severity is such that portions of fields may be left unharvested (Arsenijević et al., 1996; Arsenijević et al., 1997; Ivanović Miroslav, 1999).

The objective of the present research was (i) identification of some of the factors that influence disease development, (ii) pathogenicity of isolates on various host plants from the family Apiaceae, (iii) susceptibility of celery cultivars to infection by S. apiicola and (iv) importance of the infected seed and infected leaf debris in the overwintering of the pathogen. The role of viable spores on the seed surface has been studied and comments are given on the effect of aging of seed in controlling the disease.

Effect of duration of leaf wetness on disease severity. Two experiments were carried out: the effect of the length of leaf wetness period on the infection at permanent temperature and at different temperatures.

In the first experiment, four leaf wetness periods were tested (24, 48, 72 and 96 h) at the temperature of 17°C. At the end of each wetness period, three plants were taken out from the moist chamber (100% r.h.), dried and placed in the growth chamber at 20°C to allow the symptom development. Lesions were recorded 15 days after the inoculation (Sheridan, 1968).

In the second experiment, 2-month-old celery plants were inoculated and incubated at five temperatures (10°, 15°, 20°, 25° and 30°C) and the light regime 14 h day - 10 h night. Immediately after the inoculation, pots were bagged to maintain a saturated atmosphere, providing leaf wetness for 24, 48, 72 and 96 h. Incidence of diseased plants and disease severity, evaluated by the number of lesion per leaf, were recorded 15 days after the inoculation.

Pathogenicity of isolates of S. apiicola and S. petroselini on different host plants

This experiment was carried out to compare the reaction of the plants of 12 species from the family Apiaceae to isolates of S. apiicola and S. petroselini. Five plants per species were inoculated with isolates SPC, PGC and TP-1, using conidial suspension of 10⁶ conidia/ml respectively. Control plants per host were sprayed with sterile distilled water. Leaf spots were recorded two weeks after inoculation.

Susceptibility of celery cvs. to infection by S. apiicola

This experiment was carried out to evaluate the suscetibility of cultivated celery cultivars in Yugoslavia. Three pots, each with two 2-month-old plants were inoculated with isolates SPC using conidial suspension of 10⁶ conidia/ml. Leaf spots were recorded two weeks after inoculation.

Survival studies

Pathogen survival was studied by placing dry celery leaf with leaf spots in cheesecloth, in the soil at 10-15 cm depth, on the soil surface and at 1m above the ground, protected from the rain. Leaf debris was placed on 20 October and sampled at 30-day intervals for seven months, from 1 March to 1 October, and inspected for viability of the pathogen. The infectivity of fungal structures on leaf debris was evaluated by the inoculation method and the spore germination test. The spore germination test was carried out by hanging drop culture technique.

Septoria-leaf spot of celery is seed-borne. We used four, three, two, one year, and two month-old celery seeds. 2 g (approximately 5000 seeds) of each samples were used for preparing the spore suspension. Seeds were suspended in 10 mL of sterile distilled water and left there for 30 minutes with occasional shaking. Drops of spore suspension were examined under the light microscope (x 480) (Marshall, 1960; Sheridan, 1966). 2 x 100 seeds were spread on a double layer of moisture filter paper in Petry dishes (100 seeds in each of 9cm dish) and left at the room temperature for 2 hours. Each seed was examined with a stereomicroscope (x 6-50) for the occurrence of pycnidia. If pycnidia were found they were picked out and placed on a microscope slide and examined with a light microscope (x 100-250) (Brodal, 1986).

The present structures of fungi on the surface of the infected seeds are insufficient for the evaluation of their infection. Therefore, it is necessary to determine the viability of the pathogen on the seed. Testing the viability of S. apiicola on celery seed was carried out in two ways: hanging-drop method (Marshall, 1960) and by inoculating of celery seedlings with seed washings (Hewet, 1968).

Effect of duration of leaf wetness on disease severity

After 24 hours, the number of spots on the celery leaves is either small or there are no spots at all. However, 48 hours after constant wetness, the number of spots increases being the largest on the plants at 25^oC. After 72 hours, the number of necrotic spots is even larger. At the temperature of 25^oC and the duration of leaf wetness of 72 hours, the strongest symptoms of the disease are recorded. By storing the plants in extremely wet conditions for 96 hours, the number of spots is either slightly increased or it is the same as compared to the previous period.

The celery leaf infection by the pathogenic fungus is unsuccessful if the plants are not set in the conditions of high wetness longer than 24 hours after the inoculation. The first symptoms of the disease are recorded at the plants stored in the moist chamber 48 hours after the inoculation. The symptoms are even more severe on the plants that spent 72 or 96 hours in the moist chamber.

Pathogenicity of the isolates S. apiicola and S. petroselini on different host plants

The results shown in Table 3 suggest that out of all the tested plant varieties of the family Apiaceae which were cultivated, medicinal and as weed, only celery could be considered the host plant of the fungus S. apiicola. The narrow specialization has also been shown by S. petroselini that was pathogenic in parsley only.

Susceptibility of celery cvs. to the infection by S. apiicola

By inoculating different celery cultivars, we recorded their high susceptibility to S. apiicola.

Survival studies

The pycnidia viability lasted over 12 months in the leaf debris stored in protected conditions. The leaf debris stored at the soil surface was the source of the infection for about 12 months. Due to the intensive microbiological processes and specific ecological conditions, the leaf debris in the soil was easily broken down. In this way, the pycnidiospores in the debris lost their viability rapidly being infectious for about eight months.

The data of the pycnidiospore germination on the infected celery seed and the rate of inoculation successfulness of the healthy celery plants by pycnospore suspension are shown in table 6. With the ageing of the seed, germination of the present pycnidiospores decreases. The pycnidiospore germination is very low in the seed that is two years old and therefore, they cannot pass on an infection. At the same time, seed germination is also decreased, but it is still within tolerant limits in the seed that is four years old.

The period of continuous leaf wetness that lasts 24 hours is insufficient for the celery leaf infection. It is necessary to last 48 hours at least. The disease intensity is even more severe if the plants are in highly wet conditions for 72 hours, when the largest number of necrotic spots has been recorded. Lacy (1994) reported that either there were no infection symptoms at the temperature of 21^oC in the conditions of constant wetness during 12 and 24 hours or there were one or two leaf spots. As for the wetness period of 36 and 48 hours, the average number of spots per leaf is higher than 13. Mathieu and Kushalapa (1993) obtained similar results. The necrotic spots were most numerous at the temperature of 25^oC and the leaf wetness period of 72 hours whereas the smallest number was at the temperature of 10^oC and the wetness period of 12 hours, and at the temperature of 30^oC and the wetness period of 96 hours. The infection was not recorded when the plants were in the moist chamber less than 48 hours. The symptoms were not manifested on the plants that were inoculated by vital spores and then stored in greenhouse conditions (Sheridan, 1968).

By inoculating of more than 60 different plant varieties from the family Apiaceae, Cochran (1932) reported that only celery was the host to S. apiicola. Tetervnikova-Babayan and Anastasyan (1967) described the species of the genus Septoria found on the plants from the family Apiaceae in the former USSR. The species of the genus Septoria on the plants from this family were highly specialised. That is the reason why S. apiicola was described only on the celery and S. petroselini on the parsley. Lacy and Homma (1981) used parsley resistance to S. apiicola by inter varietal crossing of the celery and the parsley. By doing this, they obtained the satisfactory resistance of one celery variety to the pathogenic fungus.

Sorauer (1952) reported the differences in the susceptibility of the celery cultivars to S. apiicola, classifying the varieties of that time as susceptible, medium susceptible and resistant. Ocho and Quiros (1989) studied the susceptibility of 144 celery genotypes to S. apiicola. They recorded significant differences as far as the intensity of the disease symptoms were concerned, and each of the genotypes had more than 25% of the necrotic leaf surface. They also found out that the wild celery varieties, Apium chilense and A. panul, had a small number of insignificant yellow leaf spots with slow pycnide development. The resistance of the above mentioned wild celery varieties was transmitted to F₁ hybrids obtained by crossing A. graveolens x A. chilense and A. graveolens x A. panul.

The duration of preserving the infectiousness of the pycnidiospores depends on the ecological conditions imposed on the plant debris. Leaf debris stored in protected conditions was only affected by temperature changes, whereas the leaf debris on the ground surface was affected by the rainfall as well. As for the ones in the soil, the intensive microbiological processes of braking down of organic matter occurred. By increasing the intensity of the ecological factors the viability of pycnidiospores decreased.

Decreasing the viability of pycnidiospores S. apiicola is directly connected to the plant tissue breaking down. The fungus retains its viability in the soil when there is no plant tissue for only six months, or up to 18 months when the plant tissue is present (Gabrielson, 1962). According to the results of Sheridan (1966), the pycnidiospores remain viable in leaf debris in the soil up to four months. Maude and Shuring (1970) reported that S. apiicola was viable under the ground longer than nine months but not after 21 months. The spores are capable of causing the infection continuously every year. Therefore, a two-year crop rotation in celery breeding is recommended. It is important to set the conditions for the intensive breaking down of the plant debris by cultivating the soil. Without this, the pycnospores rapidly lose their viability.

Aging of the seed causes the germination of the possibly present pycnidiospores to decrease. Seed spores lose their viability after two years (Maude, 1996). Gabrielson (1962) reported that the fungus was not viable when the seed was older than one year. Hewett (1968) obtained the infected seedlings from the seed, which was one year old. Sheridan (1966) did not report viable spores in the seed 10-12 months old, whereas Maude (1964) found that the spores were viable on the seed 11 months old.

The highest number of leaf spots was recorded at 25°C for 72 hours and the lowest at 10°C and 30°C. Lesions formed as early as 9 days after inoculation following wet periods of 24 - 48 hours. The viability of the pathogenes in leaf debris lasted within the period of eight to over 12 months. The pycnides viability on the celery seeds lasted about 14 months. The isolates of the pathogens showed highly specialised. Isolates of S. apiicola caused leaf spot in celery, and isolates of S. petroselini in parsley.

1. Arsenijević M., Petrović T., Knežević T. (1996): Pegavost lišća i plamenjača celera (Prouzrokovač Septoria apiicola Speg.). Biljni lekar, 4: 523-525.

2. Arsenijević M., Petrović T., Knežević T. (1997): Septoriozna pegavost lišća i plamenjača peršuna (Petroselinum hortense Hoffm.) - Prouzrokovač Septoria petroselini (Lib.)Desm. Biljni lekar, 1: 45-48.

3. Brodal G. (1986): Method for detection of Septoria apiicola and S. petroselini on seeds of celery and parsley. National Agricultural Inspection Service, Seed Testing Station, As, Norway, 1.

4. Cochran L.C. (1932): A study of two Septoria leaf spots of celery. Phytopathology, 22, 791-812.

5. Gabrielson R.L. (1962): Survival of the Celery late blight organism. Phytopathology, 52, 361.

6. Hewett P.D. (1968): Viable Septoria spp. in celery seed samples. Ann. Appl.Biol., 61, 89-98.

7. Ivanović Miroslav (1999): Biološke odlike Septoria apiicola (Speg.) parazita celera. Magistarska teza, Poljoprivredni fakultet, Zemun, 1-74.

8. Lacy M.L. and Homma S. (1981): A source of resistance in Celery to Septoria leaf ("late") blight. Phytopathology, 71, 234.

9. Lacy M.L. (1994): Influence of wetness periods on infection of celery by Septori apiicola and timing of sprays for control. Plant Disease, 78, 975-979.

10. Lacy M.L., Berger R.D., Gilbertson R.L. and Little E.L. (1996): Current Challenges in Controlling Diseases of Celery. Plant Disease, 80, 1084-1091.

11. Marshall G.M. (1960): The incidence of certain seed-borne diseases in commercial seed samples, III. Septoria leaf spot, or blight, of celery. Ann. appl. Biol., 48, 27-33.

12. Mathieu D. and Kushalappa A.C. (1993): Effects of temperature and leaf wetness duration on the infection of celery by Septoria apiicola. Phytopathology, 83, 1036-1040.

13. Maude R.B. (1964): Studies on Septoria on celery seed. Ann. Appl. Biol., 54, 313-326.

14. Maude R.B. and Shuring C.G. (1970): The Persistence of Septoria apiicola on Diseased Celery Debris in Soil. Plant Pathology, 19, 177-179.

15. Maude R.B. (1996): Longevity of Seedborne Organisms. In: Seedborne Diseases and Their Control. CAB International, 32-43.

16. Ochoa O. and Quiros C.F. (1989): Apium wild species: novel sources for resistance to late blight in celery. Plant Breeding, 102, 317-321.

17. Sherf A.F., Macnab A.A. (1986): Septoria apiicola (Speg.). 166-169, In: Vegetable diseases and their control. John Wiley & Sons, NewYork.

18. Sheridan J.E. (1966): Celery leaf spot: Sources of inoculum. Ann. |Appl. Biol., 57, 75-81.

19. Sheridan J.E. (1968): Conditions for infection of celery by Septoria apiicola. Pl. Dis. Reptr., 52, 142-145.

20. Sorauer P. (1952): Septoria apii. in: Handbuch der Pflanzenkrankheit. Band III. Paul Parey in Berlin und Hamburg, 463-465.

21. Tetervnikova-Babayan D.N. and Anastasyan B.G. (1967): On species of Septoria on edible umbelliferous plants in the USSR. Mycologi and Phytopathologia, 1, 234-241 (in Russian).