New disease found in wheat crops in New Zealand

Farmers have a new battle on their hands as a new disease has been found in wheat crops in South Canterbury, New Zealand.

Lesions on upper leaves similar to speckled leaf blotch have been reported over the past two seasons late in the life of wheat crops, leading to significant reduction in green leaf area.

A sample of Wakanui wheat has been identified by Ian Harvey of Plantwise to contain Tan Spot (Dresclera tritici-repentis), a disease that has not been seen in Canterbury for over 30 years. In fact, a sample of the disease on flag leaves have been sent to the national herbarium in Auckland as Mr Harvey believes it will be the first official record of the fungus on plants in the field in NZ.

Tan Spot lesions are similar to those associated with Speckled leaf blotch (Septoria triticii) however do not have characteristic small black spots (pycnidia).

Mr Harvey notes that while the two diseases have similar lesions the conditions for development are very different. Speckled leaf blotch favours cold wet conditions and develops relatively slowly, while Tan spot has a cycles quickly and enjoys hotter temperatures. Tan spot is readily dispersed in air currents so is spread thoughout a crop quickly after initial infection occurs.

Tan spot will survive on crop stubble, therefore second-year wheat crops are likely to be especially susceptible to the disease.

Little is known at this stage about the most effective control options. Due to the short latent period and aggressiveness of the Tan spot disease, current standards may not be sufficient to adequately control the disease. Indeed, the sample pictured came from a crop with a standard Amistar/Opus fungicide programme.

Bayer will be conducting fungicide trials to assess the efficacy of a number of products against the disease in the future.

On the lower leaves, speckled leaf blotch is dominant, the leaves below the flag is a mixture of lesions while on the flag, it is almost all tan spot.
This would appear to reflect well the biology of the two anomorphic causal fungi.

SLB – long latent period and water-splash dispersal (of the Septoria phase)
TS – Short latent period and readily dispersed in air currents (the Drechslera phase). Also possibly higher temperature tolerance.