Conventional and organic cropping systems at Suitia V: Cereal diseases

. The occurrence of diseases on barley and winter wheat was surveyed in a field experiment comparing four conventional and four organic cropping systems in 1982—88. On barley, foliar diseases were of minor importance regardless of the cropping system. On winter wheat, powdery mildew (Erysiphe graminis), yellow rust (Puccinia striiformis) and leaf blotch (Septoria nodorum) were more prevalent in conventional than in organic cropping systems. Root and foot rot diseases (Bipolaris sorokiniana, Fusarium spp. and Gaeumannomycesgraminis) were frequent on barley and winter wheat in each cropping system. B. sorokiniana infected stem bases and roots of barley more frequently in organic than in conventional cropping systems. During the first years of the study, a serious epidemic of G. graminis was recorded in certain organic cropping systems.


Introduction
Control of diseases in organic farming is primarily preventative rather than curative. Giving up fungicides makes organic cropping systems vulnerable to certain diseases, especially during the conversion period. Diseases can be controlled by carefully planned, diverse crop rotation, balanced nutrient supply and by use of resistant varieties (Marland 1989).
Soil-borne root and foot rot diseases, B. sorokiniana, Fusarium culmorum (W.G. Sm.) Sacc., F. avenaceum (Fr.) Sacc., (teleomorph, Gibberella avenacea R.J. Cook), Gaeumannomyces graminis (Sacc.) von Arx & Olivier and Rhizoctonia spp., are common especially in conventional cereal-intensive cropping in Southern Finland (Mäkelä and Parikka 1980). They are controlled by seed dressing, crop rotation and management of energy resources in soil (Garret 1970). Organic amendments to soil increase the microbial activity, thus frequently decreasing soil-borne diseases. Cereal root rot pathogens are able to colonize organic substrates incorporated into soil, which is occasionally reported to increase their inoculum potential (Linderman 1989).
Very few studies comparing conventional and organic farming systems deal with disease problems (Lampkin 1986). The objective of this study was to survey cereal diseases in different conventional and organic cropping systems (Hannukkala et al. 1990), with special emphasis on foot and root rot diseases.

Material and methods
Diseases on barley and wheat were surveyed in a field experiment comparing four conventional and four organic cropping systems. The conventional cropping systems were managed with high chemical inputs, while no chemical fertilisers or crop protection were allowed in the organic cropping systems. The detailed experimental design is given by Hannukkala et al. (1990) pp. 295-307 in this issue. The cropping systems, outlines of their fertilisation and the symbols used in the text and tables are: Conventional cropping Al = barley monoculture, N-P-K fertilisation A 2 = cereal production, N-P-K fertilisation A 3 = diverse plant production, N-P-K fertilisation B = cattle farm, N-P-K fertilisation and slurry Organic cropping Cl = plant production, plant material composted C 2 = plant production, plant material not composted Dl = cattle farm, slurry composted D 2 = cattle farm, slurry not composted The occurrence of diseases on barley was studied in 1982, 1985 and 1988, when barley was grown in all cropping systems. Diseases on winter wheat were surveyed in 1983 and 1986, respectively. The incidence of smuts and of foliar diseases was checked visually every second week until growth stage 83 (Zadocks et al. 1974). If more than approximately 1 % of plants had symptoms on their leaves, a randomized sample of 50-100 plants was collected from each plot. The diseased leaf area of three upper leaves of individual plants was estimated using the assessment keys of James (1971).
The occurrence of root and foot rot diseases was analysed at growth stages 75 (milk stage) and 95 (harvest) (Zadocks et al. 1974). A randomized sample of 50-200 plants was collected from each plot. Plants were washed under running tap water and rated for disease.
Diseased stem bases were cut into pieces about 2 cm long. They were surface sterilised for 1 minute in 1 % sodiumhypochlorite (NaOCl) solution, 10 seconds in 96 % ethanol and rinsed with sterile distilled water. Surface sterilised stem pieces were left to dry for [3][4] hours and transferred to CMA (Cornmeal agar, Difco Laboratories, Michigan, USA) to which 200 ppm streptomycin sulfate was added to inhibit bacterial growth. Plates were incubated at room temperature (18 -20°C) at diffuse daylight and examined for fungal identification after 7 and 20-25 days. When necessary, tips of fungal hypha were transferred to PDA (Potato dextrose agar, Difco laboratories, Michigan, USA) for further identification.
To identify the fungi inhabiting the root system, a randomized sample of 50 root pieces about 1 cm long was taken from plants after disease rating. Root pieces were surface sterilised, transferred to CMA, incubated and examined as described above. In 1985 and 1986, an additional root sample (50 root pieces/plot) was surface sterilised as above and transferred to media selective to Bipolaris sorokiniana (Reis 1983). Another additional sample of 50 root pieces/plot was surface sterilised with silver nitrate solution (1 % AgNO s ) according to the method of Stetter and Leroul (1979) and incubated in media selective for Gaeumannomyces graminis (Juhnke et al. 1984). The germination of harvested seed and fungi associated with germinated and ungerminated seed was analysed by a blotter test (de Tempe 1963). 4 The experimental data were analysed using SPSS Inc. and SAS Institute Inc. statistical packages. The complete model for testing differences between means and frequencies consisted of four main factors: group of cropping systems (conventional vs. organic cropping), individual cropping systems nested within group (treatment), year (repeated observation) and replicates (blocks) (Hannukkala et al. 1990). Individual cropping systems were often ignored, and the final model included three main factors: group of cropping systems, year and replicate. The models were tested either using the analysis of variance procedure and the Tukey test or the maximum likelihood method to estimate parameters for log-linear models (CATMOD-procedure for categorical data modeling, SAS Institute Inc.), depending on the type of data.

Diseases on barley
Seed-borne diseases, loose smut and barley stripe, were detected in the experiment only occasionally. The average number of diseased plants was less than 1 per 10 square metres in all cropping systems. The seed harvested from conventional cropping systems germinated well. In organic cropping systems, the percentage of normally germinated seed was considerably lower in 1985 and 1988, which indicates high infestation of seed with B. sorokiniana (Table 1).
There was only a trace of foliar diseases in years when barley was grown in all cropping systems: net blotch and scald were found occasionally regardless of the cropping system. In 1984, stem rust (.Puccinia graminis Pers.) and brown rust ( Puccinia recondita Rob.: Desm.) attacked barley late in the growing season. At growth stage 75, 42 % of stems were infected. In 1987, when barley was grown only in conventional cropping systems, there was a heavy attack of net blotch.
On the average, 35 % of barley plants showed symptoms on stem bases. In 1982,  (Table 3). Statistically significant differences in root discoloration between replicates were found (F--value 5.97***). Essentially the same pathogens were iso- comparison of two means within one year A-B comparison of grand means of cropping system lated from roots as from stem bases. The number of B. sorokiniana infections was considerably higher in organic than in conventional cropping each year (F-values 1982: 54.32***, 1985: 110.02***, 1988: 28.90***). The roots were most severely infected in the cropping systems C 2 and D 2, where uncomposted plant material was applied (Fig. 1). Yearly variation in the number of Fusarium spp. and G. graminis infections was considerable (F-values 379.57*** and 54.97***, respectively). The highest counts for both fungi were recorded in 1982 and 1985. Cropping systems usually had no statistically significant effect on the occurrence ofFusarium spp. and G. graminis. However, in 1982, G. graminis was recovered twice as often on roots in organic as in conventional cropping (F-value 7.l4***) ( Table 3).
Diseases on winter wheat 65. The mean leaf area covered by the pathogen was approximately 0.5 %. In organic cropping systems, less than 5 % of plants had mildew on one of the three upper leaves (Fig. 2).
In 1986, leaf blotch and rusts (Puccinia striiformis Westend. and P. recondita) were common in winter wheat, while powdery mildew was of minor importance. The average percentage of plants with one of the three upper Loose smut and other primarily seed-borne diseases were rare in all cropping systems. In 1983, powdery mildew attacked winter wheat in conventional cropping systems, where more than 50 % of plants had symptoms on at least one of the three upper leaves at growth stage leaves infected with leaf blotch and yellow rust was higher in conventional than in organic cropping systems. Leaf blotch was most prevalent in cropping system A 2, where spring wheat had been grown in 1984. The highest counts of yellow rust were recorded in cropping systems A 3 and B, but the deviation between individual plots was considerable.
Brown rust infected, on the average, 10 % of plants in all cropping systems (Fig. 2).
Average root rot severity was much higher in 1983 than in 1986 (23 and 8, respectively, F = 889.59***). Cropping systems had no statistically significant effect on root rot severity (F = 0.05). The most common pathogens isolated from roots were F. avenaceum, F. culmorum and G. graminis. Also B. sorokiniana and Rhizoctonia-species were found occasionally. In 1983, G. graminis was extremely frequent on roots in organic plant production (Cl and Cl), while the counts were lowest in organic cattle production (D 1 and D 2). Fusarium species were more frequent in conventional than in organic cropping systems (F = 33.11***). In 1986, there were no statistically significant differences in the occurrence of root rot pathogens between individual or any groups of cropping systems (Table 4).

Discussion
Seed-borne diseases were of minor importance during the experiment even in organic cropping systems where no chemical seed dressing was made. This is probably due to the high quality of commercial seed used throughout the experiment. Poor germination and severe pathogenic contamination of seed harvested in organic cropping systems indicate increasing risk of seed-borne diseases on actual organic farms where domestic seed is used year after year.
The occurrence of foliar diseases is highly dependent on weather during the growing season. Disease incidence was too low to make any comparisons between cropping systems in the relatively dry summers of 1982, 1985 and 1988 (Hannukkala et al. 1990), when barley was grown in all of them. The foliar diseases attacked winter wheat more seriously in conventional than in organic cropping. In 1983, the control threshold of powdery mildew was exceeded (Kurtto 1986). The crop was, however, not sprayed, because no fungicide was registered for that purpose in Finland. More severe epidemic of powdery mildew in conventional than organic farming has been reported by El Tin and Landes (1988). In their study, higher Septoria-blotch infestation was recorded in organic than in conventional farming. Susceptibility of plants to foliar diseases is known to increase with an increasing rate of nitrogen application (Huber and Watson 1974). The lower incidence of powdery mildew in organic cropping can also be explained by the beneficial effect of slow-acting nitrogen fertilisers (El Tin 1986).
The occurrence of soil-borne foot and root rot diseases and pathogens was of roughly the same level as that reported by Mäkelä and Parikka (1980) in conventional farms in Southern Finland. During the first two years of the experiment, when the crop sequences were principally the same in all cropping systems, the heavier foot and root rot infestation occurring in organic cropping systems was probably caused by the low nutrient level and type of manures applied. Towards the end of the experiment, the differences between cropping systems decreased, which indicates that the effects of diverse crop sequences compensated for the effects of nutrient deficiency in organic cropping.
Nutrient deficiency has been reported to increase G. graminis and B. sorokiniana (Huber 1981). Severe injuries caused by G. graminis have been reported after green manuring crops, such as subterranean clover (Butler 1959). Extremely high counts of the pathogen were also recorded in this study on winter wheat in organic cropping in 1983, when the preceding crop was barley undersown with subterranean clover, while counts were low in organic cropping where slurry was applied. However, the effects of organic nitrogen on the survival of G. graminis are very complicated (Huber 1981). The high infestation of B. sorokiniana in organic cropping systems can partly be explained by the use of barley straw as the raw material for the organic manure applied. The status of fungal contamination of the straw was not analysed.
The results of the experiment indicate that the main problem in organic cropping is how to ensure an adequate nutrient supply. As no chemicals are used for seed dressing, it is crucial to pay special attention to the quality of seed. Also the organic amendments may increase disease incidence, a fact which emphasises the importance both of hygiene and of the origin of the raw materials used for organic manures. Siemenlevintäisiä tauteja, lentonokea ja ohranviirutautia todettiin kasvustoissa hyvin vähän. Peittauksesta luopuminen luonnonmukaisissa viljelyjärjestelmissä ei johtanut siemenlevintäisten tautien lisääntymiseen, sillä kokeen kylvösiemenenä käytettiin vuosittain hyvälaatuista kauppasiementä. Siemensadosta tehdyissä idätyksissä luonnonmukaisesti viljeltyjen siementen itävyys oli selvästi heikompi kuin tavanomaisesti viljeltyjen. Lisäksi luonnonmukaisesti viljellyissä siemenissä esiintyi hyvin runsaasti ohrantyvi-ja lehtilaikkutautia (Bipohris sorokiniana).