The abundance of green plant material is dependent on summer and early autumn rainfall. Awareness and management of abundant green material surviving through the summer and the incidences of pests and diseases associated with it, can aid in planning appropriate management of these before or after winter crops are sown. In particular, early control of weeds and crop volunteers to create a fallow before sowing can greatly reduce disease and insect pressure on emerging crops and conserve valuable soil moisture. For many pest and disease problems, the greatest damage is done or can develop when they occur early in the life of the crop.
A green bridge provides a host for pathogens and the build up of these pathogens early prior to crop emergence can trigger disease epidemics later in the season. Some of the common diseases that are affected by a green bridge are:
Cereal rusts (Puccinia species) only survive on living host plants. Unlike many other fungal pathogens, they have no dormant phase or any ability to feed on dead plant tissues. They are highly specialised for particular hosts. These factors act to reduce their success in typical seasons that follow a dry summer/autumn where survival with host plants is rare. Their success as parasites is largely due to their capacity for rapid population increase through repeated short generation cycles of around two weeks. The presence of a green bridge results in much higher initial populations from which to build explosive population increases.
Greatest impact from early rust pressure is in cropping zones where summer and autumn host survival overlaps with early crop establishment opportunities. This leads to regional differences in rust severity particularly early stripe, leaf or stem rust impacts in wheat, and leaf rust in barley. In areas where volunteer hosts die before new crops are established, then there are likely to be sporadic and less intense outbreaks.
The impact of rust diseases in crops is proportional to the duration of the epidemic, with early rust resulting in severe disease and greatest losses. By killing wheat and barley regrowth, thus destroying the green bridge, significant opportunity exists to minimise local threat from these diseases.
Root lesion nematodes
Root lesion nematodes (RLN) (Pratylenchus species) are widely distributed in WA cropping soils. They have two phases, either parasitism of plant roots or moisture stress induced dormancy. They do not feed on dead roots or organic matter. Their success as parasites is due to their ability to attack a wide range of both weed and crop species hosts.
When soil is so dry that no living roots are available, RLN become dormant and survive in a dry form in soil or in protective dead root tissue. As soils become wet, the nematodes become active and attack roots growing nearby. If this cycle of activity commences in summer, nematodes penetrate and feed on the roots of weeds and crop volunteers and reproduce. Their life cycle may be complete in four to six weeks with several cycles of increase possible before weeds are killed just before winter crop sowings, hence establishing population levels damaging to crop production. Wet soils with plant roots from the green bridge favour RLN survival and increase.
By contrast, wet soils without plant roots favour RLN decline. In the absence of living roots, as soil moistures fluctuate, nematodes are highly vulnerable to cycles of desiccation and reactivation. Even in continually wet soils survival in the absence of plant roots is likely to be jeopardised.
Rhizoctonia bare patch
Rhizoctonia solani causes root disease resulting in the development of bare patches in crops of cereals, legumes and in pastures. It survives as fine fungal threads in organic matter. Following summer/autumn rains the fungus grows out of this material to infect the roots of young seedlings.
Rhizoctonia is likely to be more severe where the fungus has been allowed to infect grass weeds or volunteer cereals prior to seeding. However, summer rainfall events of at least 20mm in the absence of weeds will reduce the inoculum levels in the soil and may reduce the disease impact. Early green bridge control is a helpful cultural practice in paddock preparation to minimise rhizoctonia bare patch.
The crown rot fungi (predominantly Fusarium pseudograminearum and F. culmorum) typically survive over summer on the debris from previously infected and colonised cereal plants. New infections occur when plants grow near infected debris and are favoured by soil moisture and susceptible hosts. Grassy summer weeds and volunteer cereals can all host the crown rot fungi over summer. Crown rot will be more severe in crops following a green bridge as levels of colonising crown rot fungi will already be high at crop emergence. Good summer rains will help with the breakdown of the previous years’ infected cereal stubble and weeds, reducing crown rot carryover, so long as grassy weed control is rigorously maintained.