The effect of disinfectants on fungal diseases of potato and vegetables

Treatments of one and ten minutes were too short for all disinfectants against fungi in peat and plant debris. The best effect was achieved with a treatment of 90 min. Sodium hypochlorite (NaOCl) was the most effective and Korsolin and Virkon S were the least effective in the control of Fusarium culmorum and F. oxysporum. Virkon S (2%) was the most effective against Mycocentrospora acerina and Phoma foveata. Soaking for 15 min and 60 min in a disinfection suspension eradicated Botrytis cinerea and P. foveata totally from the contaminated plastic pots. Fusarium spp. were the most difficult fungi to disinfect and these were best controlled with formaline, lobac P, Menno-Ter-forte and sodium hypochlorite. lobac P, formaline, Menno-Ter-forte, Taloset and Virkon S were the most effective disinfectants against club rot (Plasmodiophora hrassicae ).Washing under running water was not sufficient to eradicate club rot. Against Rhizoctonia -induced dampingoff of cauliflower the most effective disinfectants were formaline and Virkon S.


Introduction
Many plant diseases, which cause a decrease of yield and yield quality, survive on plant debris, growth substrates, mull debris, and greenhouse and storage structures for long periods.Therefore it is essential to use disinfectants to prevent the spread of diseases by those routes.Disinfection of soil substrate is not at present topical, because new peat or other currently used substrates are free of dis- eases.
Grey mould (Botrytis cinerea) is a common dis- ease in vegetables and ornamental plants grown in greenhouses as well as in vegetable storages (Heinze 1974, Dennis 1983).Fusarium spp.are also commonly encountered in vegetables.They cause Fusarium rot (Fusarium avenaceum and F. solani var.coeruleum) of potato (SEPPÄNEN 1983), wilt disease (Fusarium oxysporum) of tomato and cucumber, as well as damping-off and foot rot (F.culmorum, F. avenaceum).Phoma spp.cause damping-off, stem rot and various leaf spot diseases in greenhouses as well as Phoma rot (Phoma foveata) of potato.Club rot (Plasmodiophora hrassi- cae) and damping-off (Rhizoctonia solani) are a major problem in cabbage cultivation, as soil-bome diseases also in seedling cultivation (Heinze 1974).Liquorice rot (Mycocentrospora acerina) has be- come a common disease ofcarrots in storage (Den-  nis 1983).
In order to prevent the spread of pathogens on a farm and from one farm to another, greenhouses.seed storages, machines and tools must be disinfected.There are several disinfectants on the market, but little is known about their applicability in plant production against important and common diseases.In 1988-1990, studies were conducted to determine the effect of disinfectants against fungal diseases of vegetables and potato.

Material and methods
The effect of nine disinfectants (Table 1) in the control of potato and vegetable diseases was tested at the Institute of Plant Protection of the Agricultural Research Centre.Mostly the concentrations recommended by the manufacturers were used in the trials.The disinfectants were diluted in water.
The test fungi used were Botrytis cinerea, Fusa- rium culmorum, F. oxysporum, Mycocentrospora acerina, Phoma foveata, Plasmodiophora hrassi- cae and Rhizoctonia solani.The names of the fungi are mainly according to Domsch et al. (1980).The fungal isolates included in the study were obtained from the collections of the Institute ofPlant Protection of the Agricultural Research Centre.The fungi were cultivated on different media depending on the fungus (Table 2).The formulas for culture media of fungi are presented in Booth (1971).
The results were tested using the analysis of variance, and significances with Tukey's test.
The effect of disinfectants on the pathogens in peat and plant debris The effect of disinfectants on the pathogens in peat and plant debris was investigated in laboratory trials.The inoculated peat was obtained by mixing one Petri dish culture of fungus ( F. culmorum and F. oxysporum) and 100 ml of water in one litre of peat.The recommended concentrations (N) of dis- infectants and a lower concentration (10 1 N) were used.Inoculated peat, 1.0 and 0.1 g, was mixed in 11 of disinfection suspension.The preparation was allowed to act for 1,10 and 100 min.Thereafter the suspension was filtered.The filter paper was      (Ko and Hora 1971)   washed with water (50 ml) and pieces of the filter paper were placed onto different agar plates de- pending on the test fungus (Table 2) (4 pieces/plate) to determine the viability of the fungi.The trial was made with three replicates.The effect of disinfectants on fungi was tested on polyethene surfaces.The inner surfaces of the plastic pots were scratched with sandpaper and con- taminated with fungus-peat inoculate.The inocu- late used was obtained by mixing one plate of fun- gus (F.culmorum, F. oxysporum and R. solani) and 100 ml sterile water in one litre of peat, or by mixing five crushed pieces (4 cm long) of infected (B.cinerea, M. acerina and P. foveata) plant and 100 ml of sterile water in one litre of peat.The contaminated pots were allowed to dry for 1-2 days.Pieces (4x6 cm) cut from the plastic pots were soaked for 15 and 60 min in the disinfection solu- tion (100 ml).They were rinsed with water and cut into pieces of 0.5 cm onto different agar plates as above.The plates were evaluated after one and three weeks.The results were calculated as efficiency percentages, i.e. the proportion of healthy pieces on agar plates of all pieces.
The effect of disinfectants on liquorice rot of carrot ( M. acerina) and P. foveata of potato was investigated in a storage environment by inoculat- ing carrots and potatoes with disinfected peat con- taining the respective fungus.One decilitre (30 g) of peat inoculated with Mycocentrospora or P horna and 2 1 of disinfection suspension were mixed.After 30 min (trial 1) or 60 min (trial 2) the peat was filtered and washed with water.The potatoes and carrots were washed and surface sterilized with 0.1% NaOCl solution for 2 min and thereafter washed with distilled water.Four holes (1 cm in diameter and 0.5 cm deep) were made in the pota- toes and three holes in the carrots using a cork borer.The holes were filled with inoculated and disinfected peat.The carrots were stored at +5°C and the potatoes at +lO-+l2°C for 4-6 weeks in the dark.Clean peat as well as water treated infected peat were used as controls.The trial was made with four replicates, ten treatments per replicate.The trial was repeated twice.The spread of the disease in the holes was evaluated in millimetres.The efficiency percentage of the disinfectants was calculated by comparing the treatments with the healthy controls and water treatments.

The effect of disinfectants on club rot (Plasmodiophora brassicae)
The club rot trials were carried out in the green- house.The containers (Vefi) were contaminated with Plasmodiophora brassicae infested soil (1.0 1 soil/1 1 water).The soil was taken from a club rot infected rape field.The containers were allowed to dry for one day before disinfection treat- ments.
The treatments were: 1= control, no washing, no disinfection; 2=washing under running water; 3=washing under running water, disinfection for 60 min.
After disinfection the containers were rinsed with water.On the next day about 130 rape seeds cv.'Kova' were sown in fertilized peat (peat com- pound fertilizer 150 g and Dolomite lime 800 g/100 1 peat).Additional lightof 6000lux was given to the plants.The trial was made with three replicates.The seedlings were grown for about 6 weeks, until the end of flowering.The severity of the infection was determined as the proportion of infected seed- lings of all seedlings.The efficiency percentage was determined by comparing the effect of the preparation with the treatment ofno washing.
The effect of disinfectants on damping-off (Rhizoctonia solani) The inoculated peat used in the trials was obtained by mixing one Petri dish culture of Rhizoctonia solani and 100 ml of water in one litre of peat.One decilitre of inoculated peat + one decilitre of fresh peat and two litres of disinfection suspension were mixed.The disinfectant was allowed to affect for 60 min.The suspension was filtered and the peat was rinsed with water twice.The fresh peat was used as a growth medium in plastic pots (1 1).A layer of 1 cm of disinfected inoculated peat was placed on fresh peat.Thirty-six seeds of cauli- flower cv.'Tanskalainen suuri' were sown in the fresh peat and covered with a 0.5 cm layer of fresh peat.The number of replicates were five.The trial was repeated three times.The substrate, temperature and light conditions were as above in the club rot trial.The conditions of seedlings were checked 20 days after sowing.The results were calculated as the proportion of healthy seedlings of all seed- lings.

Results
The effect of disinfectants on pathogens in peat and plant debris NaOCl was the most effective in the control of Fusarium culmorum when peat (1.0-0.1 g) was mixed in disinfectant.Also Menno-Ter-forte per- formed well.Korsolin was the weakest preparation (Table 3).NaOCl was effective also against F. oxysporum.The 100 min treatment time with Menno-Ter-forte, formaline and Virkon S was sufficient to disinfect the smallest amount of peat (0.1 g).Korsolin was the weakest preparation (Table Table 5.The effect of disinfection against fungi on the surface of plastic pots contaminated with fungus-peat mixture.  4.Lower than recommended concentrations were ineffective against F. culmorum and F. oxy- sporum. Soaking of contaminated (fungus-peat suspension) plastic pots for 15 min in the disinfection suspension was sufficient to eradicate Botiytis cin- erea and P. foveala.Formaline, lobac P, Menno   Ter-forte and NaOCl were the most effective disinfectants against F. culmorum, F. oxysporum and Rhizoclonia solani.Ipasept, Korsolin and Sanisept were the weakest disinfectants on polyethene sur-faces contaminated with fungus-peat mixture (Table 5).
in the trials where disinfected peat containing Mycocentrospora acerina was tested on the carrots, the most effective disinfectant was 2 % Virkon S.
Menno-Ter-forte had a moderate effect.The effect of other disinfectants was weak (Table 6).In the potato trial on Phoma foveala Virkon S (2 %) was superior in efficacy to the other disinfectants.Ta- loset and Ipasept were the weakest preparations (Table 7).
Table 8.The effect of disinfection on club rot (Plasmodiophora hrassicae) from contaminated plastic containers.The containers were washed with water before disinfection.Rape was used as test plant.in columns marked with the same letter do not differ at P=0.05.* = P 0.05, ***= PO.OOI The effect of disinfectants on club rot (Plasmodiophora brassicae) Washing under running water without a brush, the plastic pots were not sufficiently free from club rot.All tested disinfectants were effective against club rot, but none of them eradicated it totally.The differences between the preparations were not significant (Table 8).
The effect of disinfectants on damping-off (Rhizoclonia solani ) Formaline and Virkon S were the most effective against Rhizoclonia induced damping-off.Menno-Ter-forte also performed well.Ipasept, lobac P and NaOCI were the weakest disinfect- ants (Table 9).

Discussion
In the present trials, 100 % effect against Fusarium oxysporum mixed in peat was achieved only with NaOCl (10 %) after 100 min exposure time.These results are opposite to those of BÖHMER (1983) who showed that NaOCl (1 %) had no effect on F.oxysporum.This is due to the higher concentration of NaOCl used in the present study.Menno-Ter-forte, Virkon S and formaline were also effective against the fungus in a small amount of peat (0.1 g/11 1 disinfectant).Also BÖHMER (1985) showed that a small amount of peat (15-30 g/1) does not decrease the effect of Menno-Ter-forte on F. oxysporum but a large amount (60 g/1) decreases the effect of Menno-Ter-forte.Under clean conditions Menno-Ter-forte (0.5 %) has been shown to inhibit the growth ofF.oxysporum conidia even after 5 and 10 min treatment time (Böhmer 1983, Brielmaier  1985) but this and other studies (Böhmer 1985)  confirm that the peat in the disinfection solution decreases the effect ofMenno-Ter-forteand a treatment time of 60 min at least should be used.According to Baandrup (1983), 2 % Korsolin inhibited the growth of F. oxysporum, but we used 1 % Korsolin and it was too weak against F. oxy- sporum.In this study, lobac P did not inhibit totally the growth of F. oxysporum and the effect of lobac P was decreased in the presence of peat.
NaOCl was the most effective against F. cuI mo- rum mixed in peat after 10 min treatment time.Menno-Ter-forte was moderately effective against F. culmorum mixed in peat when used at the re- commended concentration.BRIELMAIER (1985), too, showed that Menno-Ter-forte prevents the germination of conidia of F. culmorum under clean conditions.When using a treatment time of 100 min, only Korsolin was ineffective against F. cul- morum.Also this study showed that a treatment time of at least 60 min should be used.
According to BÅNG (1987a, b), formaline is ef- fective against Phoma foveata and F. solani, but lobac P is not.In the present trials, lobac P was a weak disinfectant against P. foveata mixed in peat, but formaline was effective against P. foveata.
In this and other studies (Sundheim 1991), for- maline was effective against Rhizoctonia solani mixed in peat after a treatment of 60 min.Also Virkon S was effective against the fungus and it was equal to formaline in disinfecting R. solani from peat but in disinfecting plastic pots Virkon S was ineffective against R. solani.In the present trials, lobac P was very effective against R. solani in disinfecting plastic pots, but in disinfecting R. solani from peat it performed poorly.The results are partly affected by the fact that lobac P caused damage to the cauliflower seedlings, which were used as test plants.Damping-off of cauliflower could not always be distinguished from the phytotoxicity injuries caused by lobac P. The use of cauliflower to indicate the efficacy of disinfectants is not the best choice because many disinfectants in peat cause phytotoxicity injuries to plants (Avikai-  nen et al. 1993).
F. oxysporum was effectively eradicated from plastic pots with formaline, lobac P, Menno-Ter- forte, Ipasept and NaOCl.Fifteen minutes exposure time was enough for formaline and Menno-Ter- forte, but lobac P, Ipasept and NaOCl needed 60 min.In these trials, Korsolin and Virkon S were effective against F. oxysporum although the effect was not perfect.
In the present study, all the tested disinfectants were effective against Botrytis cinerea when plastic pot surfaces were contaminated with a fungus and peat mixture.A treatment time of 15 min was suffi- cient for all disinfectants except Korsolin.In this and other studies, formaline was effective against B. cinerea (Hortica 1985, JOHANSSON 1985, Sund-HEIM 1989, 1991).Menno-Ter-forte has proved effective against grey mold growing on pieces of paper or on synthetic cloth, but not on pieces of wood (Sundheim 1991, Koponen  According to SUNDHEIM (1991), Virkon S (1 %) is not effective against grey mold, but this study showed that it is a very good compound in disinfect- ing plastic pots contaminated with fungus and peat.
In disinfecting plastic pots all disinfectants were effective against P. foveata after a treatment of 60 min.According to BÅNG (1987a, b), formaline is effective against P. foveata.The result is in accord- ance with this study.In greenhouse trials imitating seedling production, 60 min treatment of plastic pots in disinfection suspension against Plasmodiophora hrassicae gave a good result although none of the disinfectants were capable of eradicating the fungus totally.Also in other studies Menno-Ter-forte (1.5 %) has been effective against P. hrassicae when plastic pots were soaked in disinfection solution for 12 h (Hadler 1988).Whereas only washing with water or spraying of the disinfectant on the surface to be disinfested was not sufficient to eradicate club rot (Hadler 1988).
The present trials confirm the fact that dirt and organic material decrease the effect of disinfectants.The dirtier the surface to be cleaned, the longer disinfection time is needed.The disinfection time should always be at least 60 min.Although 15 min or less is sufficientin laboratory tests to eradicate the; pathogen, it is not necessarily sufficient in practice.
Virkon S, lobac P and NaOCl are effective disin- fectants against many potato pathogens, e.g.Rhizoctonia solani, Phoma foveata and Fusarium spp.lobac P can be used also for bacterial ring rot of potato (Clavihacter michiganensis subsp.sepedonicus) and black leg ( Erwinia carotovora subsp.atroseptica) (Koponen et al. 1992 a). lobac P, Menno-Ter-forte and Virkon S are good alternatives to those who want to avoid formaline in disinfection of plastic pots from Plasmodiophora hrassicae.
Formaline and Virkon S are effective against R. solani when organic material is present, but plastic surfaces may be disinfected with lobac P, Menno-Ter-forte, NaOCl and formaline.
The disinfectants, their active ingredients and con centrations recommended by the manufacturers.

Table 2 .
The growth media used in the fungal cultures.

Table 6 .
The effect of disinfection on Mycocentrospora ac- erina in peat debris.The tests were carried out on carrots.

Table 7 .
The effect of disinfection on Phoma foveala in peat debris.The tests were carried out on potato tubers.

Table 9 .
The effect of disinfection on Rhizoclonia solani in peat debris.Cauliflower was used as test plant.