Summer plantings can pass on some nasty rot to winter crops

Summer crop choices are complex because of their variable impact on pathogens which cause winter cereal diseases like crown not, common root rot, and the two main types of root lesion nematodes.

They also have a variable effect on what are commonly regarded as beneficial soil biota, especially arbuscular mycorrhizal fungi (AMF).

AMF allows some crops to more effectively use soil water and nutrients.

Authors of the study were Steven Simpfendorfer (NSW DPI), Lindsay Bell (CSIRO), Brook Anderson (CSIRO), Darren Aisthorpe (DAFQ), Mike Nowland (DPI), Jon Baird (DPI) Andrew Erbacher (DAFQ), Kathi Hertel and Greg Brooke (both DPI) which was conducted with co-investment from the Grains Research and Development Corporation (GRDC).

Eight research sites were established in 2015 and have been continued for six years to test a range of different farming systems in different environments across northern NSW, southern and central Qld.

Four sites in NSW (Liverpool Plains; Narrabri; Trangie - red and grey soil) and for are located in Qld (Billa, Pampas, Mungindi and Emerald).

Also read: Developer of wheat crown rot resistance - Dr Phillip Davies

AMF beneficial fungi colonise roots of host plants. Low levels of AMF have been associated with long-fallow disorder in dependent summer (cotton, sunflower, mungbean and maize) and winter (linseed, chickpea and faba beans) crops.

Wheat and barley are considered to be low and very low AMF-dependent crops respectively, but they are hosts and it is generally suggested they be grown before sowing AMF-dependent crops to elevate AMF populations.

In the research Mungbean had the highest increase in AMF populations, while sorghum was the lowest. Millet, often grown as a short cover crop for restoring ground cover over summer, along with maize, cotton and sunflowers had a moderate effect on increasing AMF levels.

Summer crops generally reduce the risk of Fusarium crown rot (FCR) for the following winter cereal crops, but there was variation in their effectiveness.

Limited observations support conclusions on the relative effect of summer crops on FCR. These include cotton and maize appear most effective at reducing inoculum loads, sorghum and mungbean results are more variable, but both generally reduced or only moderately increased FCR inoculum levels.

Root lesion nematodes are microscopic parasites that feed on crop roots. Two important species in eastern Australia are Pt (Pratylenchus thornei) and Pn (P. neglectus). Pt is more important in high clay content soils in northern NSW and southern Qld while Pn is generally more prevalent in lighter soil types in south-eastern Australia. Root damage restricts water and nutrient uptake causing yield loss in intolerant winter cereal and chickpea varieties.

In the research Pt densities were sufficient to examine the effect of summer crops on their levels. Summer crops vary in Pt susceptibility. Sorghum, cotton, millet and sunflower are moderately resistant-resistant (MR-R). Maize is susceptible-moderately resistant (S-MR) while mungbean is susceptible-moderately resistant/moderately susceptible (MRMS). Mungbeans resulted in the highest increase in Pt populations, and sorghum the lowest.

Common root rot (CRR) mainly infects sub-crown internodes of winter cereals, reducing primary root efficiency in susceptible wheat and barley varieties.

It has increased over the last decade with adoption of earlier and deeper sowing exacerbating infection.

Sunflowers were a non-host. Although limited observations were available from the research discussed here, it supports the overseas study. Mungbean, sorghum and maize appear generally to increase populations. Cotton, not included in the overseas study, also appears to increase common root rot risk.

Full paper details are available from "DPI Northern Research Results 2021".

Next week: Avoiding insect resistance to pesticides.

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