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Silvering is also referred to as chimera or, when the symptoms occur in the top of the plant, head silvering or white head (the Dutch translation is Witkop, Wi being the code for resistance to this disorder). It has long been the most serious disorder of glasshouse tomatoes of more northerly latitudes because the more serious problems which can occur do so when the crop is well established. Most varieties cultivated today are resistant to this disorder, but there are a number of notable exceptions:
Silvering is the result of insufficient development of a certain layer (palisade layer) of cells in the leaf at an early stage. This creates air spaces in the leaf which gives the characteristic coloring of this disorder.Symptoms can occur at any stage of the crop and can be found on the leaves, stems, and flowers. On the stems they can be stripes of varying number and width, on the leaves they are discrete patches of various sizes. If the flowers are affected they become partially or totally sterile. Any fruit developing will be deformed. Silvering really only becomes a problem to the grower when it occurs in the primary meristem (i.e. the growth tip) causing the whole shoot to become grey-green in color. This gives the plant the characteristic silvering of the head. In a November sowing, symptoms of silvering become visible on the leaves between the 3rd and 5th truss. These symptoms are however, created 5-6 weeks earlier! An abnormal cell occurs in the meristem and during the further course of the crop continues to multiply until, many weeks later, they may take over the whole meristem. The more severe head symptoms therefore do not appear before the 6th truss, generally increasing in frequency between the 6th to 12th truss. Afterwards fewer and fewer plants become affected and some become normal again. Up to 5-10% of the crop may be affected. Factors causing silvering, and prevention The research into silvering more or less stopped towards the end of the 70,s. From this research no satisfying explanation for the conditions which lead to silvering emerged. What did come out of the research was that winter sown crops are more susceptible than later sowings. The highest incidence of silvering occurred in crops sown in October, decreasing to virtually nothing in December sown crops. Further research showed that the most important environmental factor which determined the extent to which silvering occurred was the temperature. Higher temperatures lead to less silvering. Experience in practice seems to lead to the conclusion that silvering occurs as a result of a sudden lowering of the temperature of the growth meristem in the head of the plant. For example during good weather (sunshine, cold) in late winter or spring, the night sky is open and there is much radiation from the greenhouse to the sky. The air temperature may well be 16C, but the temperature in the head of the plant may descend to 10-12C. Especially vulnerable are low greenhouses because the temperature just underneath the glass is lowest. Also growers who use thermal screens must be very careful not to open the screen too quickly to prevent the cold air (above the screen) "falling onto the crop. Finally growers must take care not to ventilate too quickly or too easily during these cold weather spells to avoid venting in cold air, which may also cause the head of the plant to cool down quickly. Silvering occurs especially under low light conditions, but not as a result of low light. As a last example can be mentioned the incidental occurrence of silvering in the middle of the summer in the South of France when a warm plastic tunnel (30C) is suddenly opened from both sides, the wind blowing through causing extra transpiration and lowering of the plant (head) temperature, resulting in silvering. Reducing silvering Measures to prevent silvering are evident from the above mentioned causes of silvering. Use pipe, screen and ventilation in such a way as to prevent a sudden "cold-fall onto the heads. Use top heating pipes underneath the gutters and windows and if a growth pipe is present, bring it into the top of the greenhouse above the head of the plant. If plants do occur with head silvering the usual way to deal with them is to take the head away (prune) and grow on an unaffected sideshoot of the same plant, or of the plant standing next to it. If the first available sideshoot is far down the affected plant, the second method, i.e. taking the sideshoot of the neighboring plant, is the best option. Taking a sideshoot from the plant which had the silvering may also result in this sideshoot silvering at a later point. |