Newsletter from Danish Research Centre for Organic Farming • March 2005 • No. 1

Intercropping pea with barley reduces Ascochyta blight on pea

By Julia Kinane and Michael Lyngkjær, Biosystems Dept., Risø

Ascochyta blight is a major disease of pea, the main grain legume in Denmark. Ascochyta blight is a general term for symptoms caused by three different fungi, Ascochyta pisi, Phoma medicaginis var. pinodella, and Mycosphaerella pinodes (Photo 1). Each of these three fungi survives winter in plant debris or on infected pea seeds. Spores produced in the spring are rain-splashed or windblown to healthy plants, where they cause new infections.

Photo 1. Ascochyta blight (caused by Mycosphaerella pinodes).

Control of Ascochyta blight

In organic farming, diseases are controlled through the use of resistant cultivars and crop management practices including cropping systems. Unfortunately there is little resistance to Ascochyta blight available in commercial pea cultivars. Therefore it is necessary to focus entirely on crop management practices and cropping systems to control the disease. Due to the lack of suitable control measures in organic farming, a considerable problem is the production of disease free seed. Due to this problem the threshold for seed Ascochyta blight levels has been raised from 5% to 20% in recent years.

Use of intercropping

Intercropping is the cultivation of more than one crop species on the same area of land. Grain legumes, such as peas, in combination with cereals complement each other in animal feeds; legumes supplying protein and cereals providing carbohydrate. Intercropping allows the simultaneous cultivation of grain legumes and cereals; they can be harvested together in a mixture and used directly as fodder or they can be separated for individial use.

Intercropping is mainly practiced in tropical areas in the world, and it is of interest to study if intercropping has a role to play in temperate organic farming. Therefore the DARCOF II funded GENESIS project was established, with the aim of evaluating the potential for increasing protein production by intercropping barley with grain legumes in an organic cropping system in Denmark. Our contribution to this project was to investigate the effect of intercropping on foliar disease levels in the field.

Intercropping reduces Ascochyta blight

The simplest (and most frequent) intercrop has two components (crops). Thus, pea plants grown in a sole crop or in a pea-barley intercrop (50: 50 replacement mixture) were screened for disease in field trials in both 2002 and 2003 (Photo 2). Ascochyta blight (caused by M. pinodes) was reduced by ca. 40% by intercropping (Figure 1).

Photo 2. Left: two component intercrop: pea and barley.
Middle: three component intercrop: pea, barley and faba bean.
Right: Four component intercrop: pea, barley, faba bean and lupin.
Click to see larger pictures.

We also monitored the disease in three and four component intercrops (Photo 2); i.e. pea intercropped with barley, lupin, and faba bean (and combinations thereof) and compared them to various two component intercrops. Here it was found that the higher the number of components in the intercrop, the greater the reduction in Ascochyta blight (Figure 2).

These positive results from the GENESIS experiments encouraged us to acquire extra funding from Fonden for Økologisk Landbrug to check if varying the amount of pea and barley in the mixture had any effect on the Ascochyta blight levels on the pea plants and on the resulting pea seed.

In 2003 pea was intercropped with barley at various proportions in a replacement design (e.g. 25% barley, 75% pea; 50 % barley, 50% pea; 75% barley, 25% pea etc). As little as 25% barley in the mixture halved the amount of foliar Ascochyta blight observed (Figure 3). Increasing the proportion of barley gave additional beneficial effect.

Reduction in the level of infected seeds

Of particular interest are the levels of disease in the seed, since production of disease free seed is so problematic in organic farming. Intercropping barley with pea also reduced seed disease levels. Unfortunately the extent of the reduction was not as dramatic as observed on the foliar plant parts (Figure 4). Nevertheless, with a very high proportion of barley in the mixture (75%), just 6% of the seeds contained the pathogen (compared to 27% of seeds infected in the sole pea crop). This is only just above the maximal disease threshold allowed in seed for seed production.

Conclusions and practical uses of this research

  • Intercropping pea with barley always reduced foliar Ascochyta blight (Mycosphaerella pinodes) levels.
  • Even small amounts of barley (25%) in the crop mixture caused reductions in Ascochyta blight on the leaves and in the harvested seed.
  • Gradually increasing the amounts of barley in the crop mixture caused an even greater reduction in disease.
  • A mixture with 75% barley could reduce level of Ascochyta blight in the seed to just 6%. Despite the high level of barley in this mixture, the barley grain could be separated from the pea. Consequently such a crop mixture could provide seed for pea production and barley for alternative uses.
  • While these results are very exciting it is necessary to fine-tune the exact crop mixture to obtain even lower levels of Ascochyta blight on the leaves, but especially in the seed.
  • Addition of further crops to the pea barley intercrop is an alternative method of further reducing Ascochyta blight levels.