The disease is caused by a unique soil-bound organism (Plasmodiophora brassicae) that is closely related to ciliate protozoans but is classified in its own taxonomic group. It causes a disease called clubroot with distinct root gall formation. Under severe attack head forming fails, with crop loss as a result. Common IPM practices used today are: crop rotation, adding lime and incorporating resistance traits into brassica plant breeding.
At the edge of innovation is the use of antagonist micro-organisms. Some antagonists form a film around the roots which prevents the pathogen from establishing. Also, some organisms induce host plant resistance, which makes the brassica plants less vulnerable.
As this disease spreads through contaminated material and soil, good hygiene practices are necessary. Soil tests can indicate if P. brassicae is present and in what quantity. Soil indexing seems to be a good IPM practice to pursue. Genetic PCR-tests are provided by commercial laboratories. Mapping these test results into a soil index could aid farmers and extension services to minimize spreading risks.
Leaf spots and blight diseases
The most common leaf spots are symptoms caused by the following pathogens: Alternaria brassicae, Alternaria brassicicola, Mycosphaerella brassicicola, Erysiphe cruceferarum, Peronospora parasitica and Albugo candida. The two latter species are quite different from the other pathogens as they are not real fungi, but are categorized as oomycetes. Knowing this difference is important as the mode of action of certain fungicides does not target one group or the other.
Leaf spot and blight diseases occur when the optimal combination of three parameters is present: the inoculum (spores), the optimal environmental conditions, and a susceptible host. The current SMART IPM techniques target either one of these parameters, or a combination. Extensive modelling using big data, the use of leaf wetness sensors and spore trapping are technological innovations commanding the most attention today.
Bottom and head rot caused by Rhizoctonia solani
The fungus Rhizoctonia is widely spread and attacks many crops. Large infections can occur when optimal conditions are met. Damping off, wire-like stems, bottom rot, head rot and root rot are possible symptoms. This pathogen is soil bound, in most soils, and survives harsh conditions in the form of so called sclerotia, small and hard granule-like bodies. In greenhouses, soil fumigation has been used for a long time against soil borne diseases. Currently IPM research focuses on different strategies of bio-fumigation. The idea is to use the sterilizing effect that the natural breakdown products from brassicas have after their incorporation into the soil.
At the leading edge in terms of development of an IPM-strategy is the use of microbiological antagonists, such as other fungi (e.g. Trichoderma sp.) or bacteria (e.g. Pseudomonas sp.) strains that compete with and suppress the pathogenic Rhizoctonia population. As soils are almost infinitely diverse, a good knowledge of the microbiological condition of the soil is an advantage when developing an application strategy using these types of biological control agents.
Black rot caused by Xanthomonas campestris
No chemical pesticides are available to manage bacterial infection of brassica crops. The IPM measures used are mainly preventive in nature, such as: good hygiene, crop rotation, use of disease-free seeds, irrigation control and resistance breeding. In addition, in greenhouses, soil sterilization using steam and disinfection are commonly advised.
Recent research covers the use of so-called bacteriophage (bacteria-eating) viruses as biocontrol agents for Xanthomonas campestris. Multiple strains seem effective, but UV degradation of the viruses is still a challenge to overcome.