Vetch disease management

Take home messages

• Grain yield losses in vetch of on average of 26% were observed in plots with no disease control.
• Selecting resistant varieties and applying foliar fungicides will reduce grain yield losses.

Background

The main diseases that affect vetch are Ascochyta blight (AB) and botrytis grey mould (BGM). Other diseases which are also known to affect vetch in the Southern region include rust, sclerotinia white mould, root lesion nematodes and rhizoctonia root rot. Research into vetch diseases is very limited. Therefore, most of our knowledge on disease management is extrapolated from studies in other crops. Newer varieties, from SARDI’s vetch breeding program, have excellent rust resistance, but improvements in disease resistance for other diseases is still desirable.

Similar to other crops, the key to vetch disease management is that the cost of disease control is not higher than the potential loss from the disease. This involves thinking of the end use of the crop (hay, grain, grazing or manure) and potentially changing this end use in seasons that are very conducive to disease. In addition to affecting grain/hay yield, the disease may affect the crop quality and palatability for stock.

This paper will discuss the diseases; AB and BGM.

Grain and biomass yield losses in vetch

During 2020, a 26% reduction in grain yield due to disease was measured in a field experiment at Nhill. In this experiment there were early infections of AB (Table 1), while BGM became more dominant later in the season. Biomass differences were not significant on the 14 October (Table 2), despite significant BGM severity observed in plots at that time (Table 1). Despite variation in grain yield between vetch varieties, the interaction between grain yield and treatment was not significant, highlighting no significant difference in grain yield losses between varieties.

With two diseases present in the experiment, the 26% grain yield losses cannot be attributed to one disease definitively. However, this experiment highlights the need to control foliar diseases in vetch if the crop is to be harvested for grain. Interestingly, despite a 0.6t/ha reduction in biomass in plots with no disease control, this reduction was not significant and will require further investigation.

Table 1. Severity of natural Ascochyta blight and botrytis grey mould infection (% Leaf area affected) in three vetch varieties at Nhill during 2020. The two treatments investigated were: 1) a complete disease control (Complete) and 2) no disease control (Nil).

^AComplete treatment had multiple fungicide applications with the aim of no disease; ^BNil treatment had no fungicides applied; ^CDifferent letters indicate a significant difference in a pairwise analysis between treatment, variety or the interaction within a disease assessment date.

Table 2. Grain and biomass yields (t/ha) in three vetch varieties (3 replicates) at Nhill during 2020. The two treatments investigated were: 1) a complete disease control (Complete) and 2) no disease control (Nil).

^ABiomass was measured 14 October 2020, at peak biomass for Morava and Timok, but Benetas had not reached peak biomass; ^BComplete treatment had multiple fungicide applications with the aim of no disease; ^CNil treatment had no fungicides applied; ^DThe variety x treatment interaction was not significant, and therefore the % yield loss between varieties is also not significant. ^EDifferent letters indicate a significant difference in a pairwise analysis between treatment, variety or the interaction within a disease assessment date.

Ascochyta blight

Ascochyta blight affects vetch crops early in the season with cooler wet conditions favouring disease development. When temperatures increase and the canopy dries out later in the season, visual symptoms of AB reduce. This is thought to be from the infected leaves dropping off and the vetch plant growth increasing, thus reducing the overall percentage of infected leaves.

There is significant variation in the resistance of varieties to AB, with Morava, Timok and Rasina being more susceptible than other released varieties (Figure 1). Consult the latest Agriculture Victoria Pulse Disease guide for up-to-date resistance ratings.

The risk of infection by AB can be reduced by choosing a more resistant variety and ensuring that there is at least three years between vetch crops. In susceptible varieties, it is thought that if severe symptoms are observed a fungicide may be warranted to prevent large biomass losses.

Figure 1. Mean ascochyta blight infection in vetch varieties at Horsham during 2020.

Botrytis grey mould

Botrytis grey mould in vetch is difficult to control due to early sowing and canopy closure. As a result of early canopy closure there is higher canopy humidity, which is conducive to disease development along with increasing difficulty of uniform fungicide coverage, even if a high-water rate is used. Although a higher temperature (>15 degrees Celsius) is optimal for BGM development, at lower temperatures the disease can develop, just at a slower rate.

Although most vetch varieties are susceptible to BGM there are several management practices which can be utilised to reduce disease severity in vetch crops, including:

1. Crop rotation

   As the same pathogens cause BGM in vetch, faba bean and lentil this needs to be considered when planning rotations to allow sufficient time for stubble breakdown between affected crops. This time will vary between rainfall zones.

2. Time of sowing

   Time of sowing can help to determine when canopy closure occurs. If sown too early and not grazed, canopy closure can occur too early, not allowing fungicide penetration into the crop when BGM starts to spread in late autumn/early spring.

3. Foliar fungicides

   The best defence against disease in vetch crops is a foliar fungicide applied just prior to canopy closure to prevent disease development. In other crops, newer released fungicide actives have provided longer protection against disease and may also provide longer protection in vetch. In conducive disease years, multiple fungicides may be required. It is important to check product registrations and withholding periods, especially when the vetch crop is to be grazed or cut early for hay.

4. Grazing
   Grazing may open up the canopy to reduce the canopy humidity and thus disease risk. This may be particularly relevant in early sown crops.

Conclusion

Vetch disease can cause large grain yield losses, with 26% observed during 2020 at Nhill. There are several disease control management practices that can be utilised, but it is always important to remember the end use of the crop (i.e., manured, grain, hay or grazed) as this will dictate the economic viability of the control options.

Contact details

Joshua Fanning
Agriculture Victoria
joshua.fanning@agriculture.vic.gov.au

Stuart Nagel
SARDI
stuart.nagel@sa.gov.au

Varieties displaying this symbol  beside them are protected under the Plant Breeders Rights Act 1994