Early-season water management and common scab control in potato crops

Irrigation programmes

Adjust your irrigation programme depending on your soil type and potato variety. Understanding specific variety requirements to optimise marketable yield and quality is essential, which firstly requires accurate and effective irrigation scheduling. Figure 1 shows the maximum soil moisture deficit (SMD) which needs to be maintained in order to achieve a good level of common scab control in different groups of varieties.

Starting the control regime

For varieties susceptible to common scab (Group 1, Figure 1), you should apply water as soon tuber initiation (TI) starts. Aim to bring the ridge of bed back to field capacity. For resistant (Group 4) varieties, a short delay (one week) can be tolerated. The top 25cm of soil should be kept moist during this period. A spade can be used to ensure water is wetting the ridge or bed around tubers. Repeat application may be needed within two to four days

What you should do:

• Apply water evenly and accurately
• Measure what you apply
• If using drip irrigation, avoid watering for too long at each application and ensure the system is fully operational before tuber initiation

The critical period for control is between one and three weeks after TI. This is useful for susceptible processing varieties and where irrigation capacity is limited.

How long to irrigate for

For Maris Piper, with a typical commercial emergence period, 31 days after start of TI. Maintain the correct SMDs for varieties and soil types (see Figure 1) for three to four weeks after TI.

Potato varieties for the salad market are at risk from common scab infection for much longer owing to the small size of tubers. Scab control for six weeks is sufficient, even in susceptible Group 1/2 varieties such as Maris Peer or Charlotte and as short as four to five weeks in more resistant Group 3/4 varieties such as Perline, Regina or Juliette.

What to avoid

• Ceasing irrigation two weeks after TI allows pathogens to multiply rapidly and can increase scab infection unless rain falls
• Avoid overwatering soils during the scab control period, particularly in the first week after TI. Excessive irrigation or soils kept above field capacity for substantial periods can aggravate other disease problems and impede root growth, leaving the crop more susceptible to drought later in the season
• Uneven or protracted emergence lengthens the control period required. This increases costs and water use, and risks higher levels of scab. Good seed management, soil cultivation, planting depth and agronomy can all increase crop uniformity and improve scab control

Figure 1