Alternaria is usually caused by the fungal pathogen Alternaria brassicae and occasionally by Alternaria brasicicola. Canola cultivars are more resistant to A. brassicicola. The severity of the disease varies between years and locations depending on seasonal conditions. The disease is favoured by warm humid conditions during spring. Yield loss is unusual and is normally associated with the shattering of infected pods. If infected seed is sown a seedling blight may occur (refer to damping-off section).
Alternaria infects all growth stages of canola plants. However, as plants mature from mid flowering onwards they are more susceptible to infection. Alternaria symptoms can be found on all parts of the plant including leaves, stems and pods. Spots on leaves and pods have a concentric or target-like appearance and are brown, black or greyish white with a dark boarder.
Lesions on green leaves are often surrounded by a chlorotic (yellow) halo. Severe pod infections may cause seed to shrivel and the pods to prematurely ripen and shatter. Stem spots are elongated and almost black. Pod symptoms of alternaria are similar to those of blackleg and the two can be difficult to distinguish in the field, (Figure 11.12).
Figure 11.12 Symptoms of alternaria on canola.
- Alternaria lesions on the leaf (note target like rings)
- Lesions on stem
- & 4 Pod lesions that can lead to premature pod shattering Photos: S. Marcroft, Harsh Raman
Alternaria spp. survive the intercropping period on infected canola stubble, on cruciferous weeds and to a lesser extent on seed. Seed infections can cause seedlings to rot (refer to damping-off section) resulting in a seedling blight that reduces plant establishment. Initial crop infections are caused by wind blown spores. Spores remain intact on susceptible plants until moisture from dew or rain allows them to penetrate into the tissue and cause a lesion. These lesions produce more spores and infections can then be spread throughout the crop by either the wind or rain, (Figure 11.13). Mild, humid conditions favour disease development and the disease cycle will continue throughout the season under favourable conditions. Hot and dry conditions interrupt epidemics as the absence of moisture greatly reduces spore production. Major outbreaks are not common in Australia as weather conditions are normally hot and dry throughout podding which is unfavourable for prolonged infection.
Figure 11.13 Disease cycle of alternaria on canola. Illustration by Kylie Fowler
- alternaria is very common in canola crops but is not usually severe enough to warrant control
- in Australia, there are no registered fungicide seed treatments for alternaria
- if pods were infected in the previous season, obtain fresh disease free seed
- in areas where alternaria is a problem select paddocks isolated from last years canola stubble as alternaria spores are easily transported in wind and can spread into areas that have not had canola for several years.
SCLEROTINA STEM ROT
DAMPING-OFF (SEEDLING BLIGHTS AND SEEDLING HYPOCOTYL ROT)