The objective of the harvesting operation is to lift the crop with the minimum of damage to the tubers prior to storage, and with the minimum amount of clay, dirt, stones etc.
The first stage in harvesting potatoes is to desiccate or “burn-off” the foliage (haulms). This is particularly important in crops for storage. In the case of crops which are being graded and sold immediately it is less critical. Crops lifted with immature or blighted foliage carry a high risk of rotting in storage.
Potato foliage and stalks need to be removed before harvesting to prevent blight infection of the tubers and to facilitate the passage of the harvester. Burning off also removes any weeds which would interfere with the working of the harvester.
Desiccation is the application of a special agrochemical designed to kill off the green foliage. The desiccant should be applied when the tubers have reached the desired size. Dig up tubers in different locations in the field to assess their size and make a decision. Allow a period of two to three weeks from the time of application of the desiccant to the start of harvesting.
This allows the tuber skins to mature and therefore be less prone to damage and disease at harvest time and in the store. Aim to harvest crops by mid - October, so that the operation can take place in relatively warm (7oc or greater) soils. Harvesting potatoes in cold wet soils will initially lead to a greater number of diseased and rotten tubers.
There are a range of chemical products on the market available for use as desiccants. A number of the most popular products are based on the chemical Diquat, which works by contact and has limited translocation in the plant.
Warning: Haulm desiccants can damage tubers if applied during or shortly after dry periods. A “dry period” definition varies with soil and crop type, so read the product label carefully before use. Some varieties may also be susceptible to a particular chemical.
Lifting and Harvesting
Healthy potatoes in the soil at harvest time are in perfect condition.
Damage to the tubers is caused by:
- Late harvesting in November and December
- Wide tyres encroaching on the ridges resulting in bruised and damaged tubers and the creation of clods.
- Setting of the digging share either below or above the bed of tubers and excessive agitation of the webs of the harvester.
- Worn and sharp edges on machinery parts.
- Allowing potatoes to drop in height in excess of 20cm, thereby causing damage.
As much as 20% of tubers are damaged at harvest time. So it pays to follow the correct procedure to ensure losses are kept to a minimum.
Haulm destruction for salads
Timing of haulm destruction for salads is more important than for other potato types as it is critical to maximise progeny tubers in the target range (e.g. 25-42mm). Over-sized tubers, particularly, have very little value. Achieving uniform spacing of seed at planting and uniform emergence of the crop will both hep to provide a uniform tuber size distribution at the time of haulm destruction.
When to carry out haulm destruction
For other potato types, some growers allow the largest tubers to grow oversize on the basis that more of the under-size tubers will come into the marketable fraction. However, larger tubers tend to be growing in size faster than small tubers and this approach does not always work. In any case some increase in size will occur during the process of haulm destruction (see below).
The most usual approach for salads is to start the process of haulm destruction when the largest tubers are at largest size in the marketable fraction in at least 2 of 3 test digs. Thus, in a salad crop with a top riddle size of 42mm, the largest tubers should be at this size when haulm destruction starts. Only where there is a market for oversize tubers is it sensible to delay beyond this timing for haulm destruction.
How much do tubers increase in size after haulm destruction starts?
There is no simple answer to this question as it depends on how rapidly tubers are growing, the soil conditions and the speed of haulm destruction. However, experience suggests that if soil conditions are dry and uptake of nutrients and water limited, tubers may increase only 1mm (or at maximum 2mm) in size, irrespective of haulm destruction method.
If the soil is damp or wet, the increase in size depends on how rapidly haulm destruction is achieved. Experience suggests pulverising the haulm tends to restrict subsequent tuber growth better than chemical desiccation. Under damp or wet conditions, as a rough guide, when pulverisation starts the haulm destruction process, increase in tuber size is generally 1-2mm but with chemical desiccation is could be 2-3mm.
These are only guides but using test dig results where tubers are placed in size bands it is possible to approximately ascertain how many might move into oversize based depending on the method of haulm destruction.
Method for test digging
For each salad crop, you need to be clear what the size specification is. Occasional examination of a few plants of each stock in a field on a weekly basis will provide an idea of when tubers are approaching the critical bulking stage (when largest tubers are within 5-10mm of top riddle size). Once this critical bulking stage has arrived, more intensive digs are required every 2-4 days.
At least three locations for test digs should be selected at random from across the stock but avoiding any poor or unusual areas in the crop. One test dig per field is not enough as you may have chosen a poor area by chance. Two test digs per field will not give a good idea of the variation in the field. More than three digs will improve confidence of an accurate result but takes more time.
Repeat digs should take place close to these first intensive dig sites so that you are digging similar crop each time. Ideally for each crop, each dig should comprise lifting at least 1m of drill and preferably 2m. Exact length of drill is not critical but it must be more than 1-2 plants to achieve a meaningful result. The minimum information from each dig is size of largest tuber but dividing the tubers lifted into appropriate size fractions will give the size distribution. Weighing the tubers in each fraction (especially the marketable fraction) adds an estimate of yield
Haulm destruction methods
There are currently three options for haulm destruction of salads
- Pulverisation followed by desiccation with carfentrazone-ethyl or diquat or both
- Desiccation using diquat followed by pulverisation followed by desiccation with carfentrazone-ethyl or diquat or both
- Desiccation using diquat followed by desiccation with carfentrazone-ethyl or diquat or both
All three options work and the time to complete haulm kill is usually similar. As indicated earlier, pulverisation probably restricts subsequent progeny tuber development more than desiccation. A water volume of at least 300 l/ha is required to effectively haulm kill a vigorously growing salad crop. Where pulverisation is carried out, aim to leave cut stems of around 20cm (8”) and do not follow up with a desiccant until leaf tissue has dried and exposed the cut stems.
A blight fungicide should be mixed with desiccants as long as green leaf or stem tissue persists. For low blight-risk situations fluazinam products can be used. Ranman Top and Infinito are good alternatives especially in higher risk situations.