Diseases

Current Status and Challenges

Disease control in horticultural brassicas

Plasmodiophora brassicae

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.

Pests

Current Status and Challenges

Pest control in horticultural brassicas

Introduction

A variety of crops are included in the term ‘horticultural brassica’ and this includes crops where the marketable part of the plant is above ground (cabbage, cauliflower, broccoli, Brussels sprout) and others where the root is the marketable part (turnip, swede, radish). These crops are grown mainly outdoors and they can be infested by a variety of pest species (at least 50 species of fly, beetle, aphid, moth/butterfly, thrips). Some of these are ‘regular’ pests and others are sporadic. These pests are currently managed mainly using pesticides, with physical control being important in some cases. There are few commercial examples of host plant resistance. The current status of IPM in brassicas was reviewed by an EIP-Agri Focus Group in 2014-16.

What follow below is a list of the most important pests.

Root-feeding fly pests

We have recently completed a review of methods of managing the of vegetable crops (see here) as part of the C-IPM FlyIPM project and this included cabbage root fly (Delia radicum) and turnip fly (Delia floralis) which are regular pests of a range of brassica crops throughout Europe. Whilst a large number of alternative management approaches have been researched, apart from insecticides, only physical barriers are used in commercial practice (crop covers, fences around fields). Monitoring and forecasting systems are reasonably well-developed for these pests but monitoring systems are not automated and monitoring and forecasting systems are not integrated. The range of insecticidal control options available to growers is diminishing. For the future, growers need ‘SMART’ ways of identifying, forecasting and monitoring infestations and of controlling the pests.

Caterpillars

Another key pest is the diamondback moth (Plutella xylostella), important globally, and again there are major challenges in finding new tools to manage this pest, as an alternative to insecticides. The diamondback moth has a great propensity for the selection of insecticide resistant strains and this provides an additional challenge. Although large infestations by diamondback moth are sporadic – at least in northern Europe – there is increasing interest in managing it more effectively due to a significant infestation in 2016 in several countries. This is a migrant species in much of Europe, overwintering in warmer climates, which makes its arrival less predictable than species that overwinter locally. One key aspect is to be able to identify the arrival of large numbers of migrant moths, and recent research has been undertaken in the UK to find ways of doing this remotely, either using information posted by citizen scientists on web sites or using automated monitoring with pheromone traps. This research has been summarised here. There is scope for both of these approaches to be made ‘SMARTER’.

Aphids

Aphids are a third important group of pests, key species being cabbage aphid (Brevicoryne brassicae) and peach-potato aphid (Myzus persicae). They contaminate the crop, cause physical damage and may transmit plant viruses. The peach-potato aphid also adapts readily to insecticide pressure through the selection of resistant strains. Aphids have a range of natural enemies, predators, parasitoids and entomopathogenic fungi and although few of these approaches are used commercially on outdoor crops, SMART application techniques might increase their effectiveness and consistency and lead to greater use by growers. Aphids disperse through flight and in several countries there is a network of traps that is used to monitor flight activity – either suction traps, water traps or sticky traps. None of these systems is automated and considerable time is spent visiting traps to service them and identifying the key species from samples. SMART approaches to these tasks would make a huge difference.

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