Crops notes September 2006

Desiccate all crops as soon as tubers reach the desired size range for your target market and aim to have all ware crops desiccated by mid September.

Late harvest increases the risk of mechanical damage, disease infection and slug attack.

Potato haulm can be defoliated by mechanical (flailing) or chemical methods using one of four chemicals: glufosinate-ammonium (Harvest), carfentrazone-ethyl (Spotlight) and the longer established diquat (Reglone) and sulphuric acid treatments.

In dry soil conditions desiccation with Reglone may be linked to an increase in stem end necrosis and vascular browning. The key to reducing the risk of vascular browning at defoliation is to desiccate the crop in a number of stages using mechanical and or chemical treatments rather than rapid desiccation using full dose rates in a single application.

The first treatment (T1) is used to weaken the haulm by desiccating the leaves, while the second treatment (T2) is used to target remaining leaves and stems.

The most important aspect of a flail and spray sequence is the quality of flailing. Chopped haulm must be channelled into the troughs, not on top of the drills and haulm choppers should have metal blades. Stems should be cut to 15-20 cm in length and should be left free from flailed leaf material.

In dry soil conditions plan to desiccate potato crops in the morning as tubers are less prone to vascular browning and bruising than in the afternoon and evening.

Glufosinate-ammonium (Harvest) should only be applied to crops, which have started to senesce. Harvest may be applied to all varieties grown for seed or ware except Kerr’s Pink.

Carfentrazone-ethyl (Spotlight) may be applied to all varieties of seed and ware potatoes and can be used in wet and dry soil conditions. Spotlight may be used for the desiccation of stems and as such is a T2 product for use as part of a sequence following flailing or chemical defoliation.

Split rather than single applications of diquat (Reglone) give growers flexibility to achieve improved haulm desiccation. Where haulms are of average size one litre/ha should be applied initially to open up the crop canopy followed three to five days later with three litres/ha to complete desiccation. Where tops are large and green it may be beneficial to use two equal applications of two litres/ha each. No more than four litres/ha of Reglone should be applied in total.

It is important to prevent blight spores infecting tubers. Growers are recommended to apply blight fungicides with good performance against tuber blight, such as Shirlan, Ranman or Electis, both prior to desiccation and in tank mix with your desiccant. Further fungicides will be needed if the haulms are still green after desiccation. At the end of the season, if there is blight in the crop, destroy the haulm as soon as possible to reduce the risk of tuber infection. Sulphuric acid gives the quickest haulm kill and is the best product in this situation, but must be applied by a contractor.

Spores of many storage diseases lie dormant in the dust and debris remaining in stores, boxes and equipment from the previous harvest. Thoroughly power-hosing or vacuuming stores and equipment can achieve significant reductions in infection before the new crop is harvested. Effectiveness can be further improved by applying an approved disinfectant after initial washing or vacuuming.

Removing excess surface moisture is of great benefit in reducing the incidence of diseases such as silver scurf and soft rots that thrive in wet conditions. Ventilation systems have been developed which force high volumes of air through boxes or bulk piles for a short period to dry the crop quickly. Dried potatoes are then cured as normal prior to storage.

Wet or damp crops should be dried as soon as possible after harvest. If fungicides are to be applied to the crop, they should be applied prior to wound healing to ensure effective disease control.

Forced ventilation systems are available to suit most needs -drying floors, horizontal ducts, and pedestals for bulk stores, tents, suction walls, posi-vent ducts, and letterbox systems for box storage.

Some damage will occur during harvesting and a short period of wound healing or 'curing' should be allowed. After drying, the crop should be held at ambient temperatures (12-15C) for a period of 7-10 days. It should then be ventilated periodically using dry outside air to remove moisture from the store. Once curing is complete the crop can be brought slowly down to the desired storage temperature.

Strategies for Barley Yellow Dwarf Virus (BYDV) prevention should start prior to sowing. Crops following grass, weedy stubbles, and set-aside can be infected by wingless aphids, which walk directly from this 'green bridge' onto the emerging cereal crop. This green bridge should be destroyed 7-10 days prior to ploughing, using glyphosate or paraquat. The cereal crop should then be sown no sooner than 14 days after herbicide application.

Some growers may have carried over seed from last year, in particular winter wheat. Many factors could influence the germination of this seed including storage conditions and seed dressings. Seed should have a minimum germination of 85 percent and preferably over 90 percent. AFBI Plant Testing Station at Crossnacreevy can test seed for germination, Tel: 028 9054 8000.

Following harvest is an ideal opportunity to carry out soil analysis. With fertiliser accounting for a considerable proportion of cereal variable costs (approximately 35-40 percent in the case of winter wheat), it is important that this input is used efficiently. In addition, fields with a low pH can be identified and rectified with applications of lime.