Haulm destruction and harvest

Haulm destruction before harvesting is often applied to facililtade harvest and/or to reduce tuber damage at harvest.
Haulm is A.) pulled, B.) cut, C.) pulverized or D.) killed using chemicals.

In general haulm destruction takes place 10-15 days before harvest. It is evident that a good skin formation after haulm destruction is quicker in a crop which is close to maturity, than in a crop which is still in full production. A more mature crop can be harvested sooner after haulm destruction than an immature crop. The harvest method used also strongly influences the length of the period needed between haulm destruction and harvesting. With manual harvesting methods this period can be short (often a few days are sufficient).

The method used has an effect on:
Haulm pulling
Haulm pulling is the most effective method to reduce skin damage at harvest.
Rhizoctonia:
Also after haulm pulling the skin sets fast, while the development of black scurf on tubers in the soil is slower than after chemical haulm killing. As a consequence it is easier to produces seed free from Rhizoctonia clean seed when haulm pulling is applied.
Virus:
When haulm pulling is applied there are less problems with virus infection of tubers. The main reason for this is that after haulm pulling there is no re-growth of the stems, which are very susceptible to virus infection. The second reason is that viruses present in the stems after a primary infection cannot reach the tubers.
Disturbance of the ridge:
With haulm pulling however the ridge is often disturbed, leaving the tubers unprotected by a layer of soil. This can cause greening of tubers, tuber moth attack and late blight attack of tubers and an exposure of tubers to high temperatures.
Haulm pulling is labour intensive.

Haulm cutting
Haulm cutting is applied to facilitate harvesting.

The method is rather good: If the main purpose of haulm destruction is skin hardening it is better to consider either chemical haulm killing or haulm pulling.

Virus and re-growth:
When crops are still rather vegetative in their growth, the skin hardening process is slow and there is the risk for re-growth of new shoots in the axial of the stems. These new shoots are very susceptible to viruses.
Rhizoctonia:
After haulm cutting it takes quite some time for a proper skin setting and there is the risk that tubers will be contaminated with black scurf.

Chemical haulm killing
One of the most commonly used herbicides is Diquat (Reglone) at a dosage of 3-5 litre per hectare. In crops with a luxuriant haulm growth one spray may not be sufficient to kill the crop and it may be necessary to repeat the spraying.
Stem end and vascular ring necrosis:
After chemical haulm destruction stem end and vascular ring discolouration may occur due to necrosis. The stem end attachment is often slightly sunken and only the vascular ring near the stem end will turn pale yellow to dark brown. In severe cases, the whole vascular ring discolours and the tuber starts to rot from the stem end (a dry rot) during storage.
The risk of this disorder occurring is particularly great if the chemicals are applied at high temperature while the plants are under high water stress. To avoid this it is advisable not to spray during daytime, when the water stress in plants is highest but to wait until late evening or early morning.
Rhizoctonia:
After chemical haulm killing it takes quite some time before the skin is properly set. During this period the tubers often become contaminated with black scurf (especially when crops are still immature). To reduce black scurf infection the interval between haulm killing and lifting of the tubers should be kept as short as possible.
Virus and re-growth:
When crops are still rather vegetative at the time of chemical killing, the skin hardening process is slow and there is the risk of re-growth of new shoots in the axials of the stems. These new shoots are very susceptible to viruses.