Mid West potatoes: seed production, pest and disease management

Disease management

Skin blemish diseases at harvest or postharvest downgrade the packouts of ware potatoes, and some reduce yield. Black dot (Figure 2) and silver scurf (Figure 3) can be particularly prominent in some years, while scab diseases occasionally occur in sandy soils (Figure 4). Control generally begins at planting.

Figure 2 Black dot skin blemish lesion which gets its name from the small black sclerotes (dots) in the lesion

Figure 3 Silver scurf on Royal blue potatoes has a silvery shine when viewed with lights and does not contain the small black sclerotes present in the centre of the lesion

Figure 4 Common scab is caused by a bacterium that produces roughly circular lesions on the tuber surface that are slightly sunken at the centre with flakes of skins surrounding the lesion

Some in-furrow or seed piece chemical applications are registered for the control or suppression of these diseases.

Make sure that tubers are at the correct temperature at planting to prevent sweating or excess surface moisture. Applying liquid fungicides to tubers that are sweating could promote seed piece breakdown. Some of these chemicals are registered for in-crop use so consider resistance management when deciding which chemical to use.

Depending on weather conditions, the crop may be susceptible to foliar diseases that could reduce overall yield. Frosts or other mechanical damage can cause entry points for diseases such as botrytis or early blight (target spot).

Figure 5 Early blight (target spot) of potatoes caused by Alternaria species. Note the concentric growth rings in the lesions.

Overwatering can lead to black leg and Sclerotinia (white mould) diseases as it increases humidity and stresses the plant, reducing its ability to naturally defend against infection.

Over-application of nitrogen fertiliser can lead to excess plant growth. Weak plant cells make crops more susceptible to disease.

Early blight is the most common foliar disease in WA  and the first signs are small circular spots that expand in concentric rings (Figure 5). Older leaves are usually infected first but the disease, if left unmanaged, can lead to severe infection that reduces the crop’s lifespan and subsequent yield.

Numerous chemicals are registered for control of foliar diseases, with several modes of action. Some work as contact fungicides, providing a barrier from infection. Others have translaminar activity where they penetrate the leaf and travel a small distance beyond the point of spray. Foliar diseases of potatoes can develop resistance to the chemicals used to control them so follow the resistance management guidelines for potatoes to prevent this from occurring. The guidelines are published by Croplife Australia.  Always read the label of the fungicide thoroughly prior to application.

Acknowledgement

Much information in this publication is based on results from a number of potato research and development  projects funded by Horticulture Australia Limited with voluntary contributions from APC Potato Producers Committee, matched by the Australian Government and with in-kind contributions from and managed by the Department of Agriculture and Food.