Chrysanthemum Diseases

The following article, which I hope, will be of interest to growers, will possibly give answers to a few questions that arise about the leaves of some cultivars during the growing season. It has been adapted from a Victorian Chrysanthemum Society Newsletter of September 1985, which was based on a previous report made in 1982 from Florida USA, to which acknowledgment is made. Assimilation photosynthesis occurs when leaves extract carbon dioxide from the air. The chlorophyll in leaves acts on the gas and separates carbon and oxygen. Oxygen returns to the atmosphere, whilst carbon plus water which is supplied by the roots of the plant, become a soluble plant food and later less soluble food starch which is useful for storing in the leaves. Under certain conditions, more sugar or total carbohydrate is synthesised than the plant can use, which will bring on High Sugar Disease in chrysanthemums. The excess sugar, or excess total carbohydrates result in the development of a range of plant symptoms. One or more may develop in any given cultivar. There are leaves which may develop into being thick, leathery, brittle and downward cupped. The leaves may also have interveinal chlorosis, necrosis and have chlorotic or nephrotic spots, as well as browning or blackening of the tips and margins of the leaves. The symptoms of magnesium deficiency may also overlap some of those caused by disease of high sugar. Other apparent symptoms, which may be related to excess sugar, are petal spots and petal necrosis on the blooms. These symptoms vary very greatly amongst different cultivars. The symptoms are more prominent during certain periods of the year and at definite periods in the development of the plant. Since light intensity and day length are major contributors to the development of this physiological disease more symptoms are apparent in December than later on in the year, say in May. Also, the development of symptoms intensifies after the plants are stopped. Pruning of any excess growth removes growing plant tissues which are using excess carbohydrates which the plant will normally synthesise. The excess carbohydrate which accumulates will eventually become toxic to the plant. Each cultivar responds according to its own genetic tolerance to the condition. Stock plants, where new growth is removed regularly are very susceptible to this disease. Constantly removing new growth induces high sugar content in the older leaves. But, do not despair, there are a number of ways that the disease and it's effects may be minimised. Reducing the intensity of sunlight on the plants by using a shade cloth which reduces up to 50% of sunlight, will minimise the chances of plants being affected, especially in the final month. Denser planting also gives protection. Symptoms are more severe on the exposed south and west sides of the beds than on the north and east sides and interiors of the beds. Therefore planting to obtain the least exposure, such as orientation of the beds on a north to south basis rather than an east to west direction is helpful. Nitrogen and magnesium nutrition can affect the high sugar disease. High levels of nitrogen temporarily alleviate the adverse effect of the high sugar, especially when nitrogen is supplied, mainly in the nitrate form rather than in the ammonium form. Magnesium deficiency can accentuate the disease. When high sugar in the plants adversely affects quality, steps should be taken to ensure an adequate magnesium supply. Growers should strive to produce plants that have flat, green flexible leaves free of any excess cupping, thickness, brittleness, crumpled shapes and physiological leaf spot.

HIGH SUGAR DISEASE IN CHRYSANTHEMUMS by George Harrington
The following article, which I hope, will be of interest to growers, will possibly give answers to a few questions that arise about the leaves of some cultivars during the growing season. It has been adapted from a Victorian Chrysanthemum Society Newsletter of September 1985, which was based on a previous report made in 1982 from Florida USA, to which acknowledgment is made. Assimilation photosynthesis occurs when leaves extract carbon dioxide from the air. The chlorophyll in leaves acts on the gas and separates carbon and oxygen. Oxygen returns to the atmosphere, whilst carbon plus water which is supplied by the roots of the plant, become a soluble plant food and later less soluble food starch which is useful for storing in the leaves. Under certain conditions, more sugar or total carbohydrate is synthesised than the plant can use, which will bring on High Sugar Disease in chrysanthemums. The excess sugar, or excess total carbohydrates result in the development of a range of plant symptoms. One or more may develop in any given cultivar. There are leaves which may develop into being thick, leathery, brittle and downward cupped. The leaves may also have interveinal chlorosis, necrosis and have chlorotic or nephrotic spots, as well as browning or blackening of the tips and margins of the leaves. The symptoms of magnesium deficiency may also overlap some of those caused by disease of high sugar. Other apparent symptoms, which may be related to excess sugar, are petal spots and petal necrosis on the blooms. These symptoms vary very greatly amongst different cultivars. The symptoms are more prominent during certain periods of the year and at definite periods in the development of the plant. Since light intensity and day length are major contributors to the development of this physiological disease more symptoms are apparent in December than later on in the year, say in May. Also, the development of symptoms intensifies after the plants are stopped. Pruning of any excess growth removes growing plant tissues which are using excess carbohydrates which the plant will normally synthesise. The excess carbohydrate which accumulates will eventually become toxic to the plant. Each cultivar responds according to its own genetic tolerance to the condition. Stock plants, where new growth is removed regularly are very susceptible to this disease. Constantly removing new growth induces high sugar content in the older leaves. But, do not despair, there are a number of ways that the disease and it's effects may be minimised. Reducing the intensity of sunlight on the plants by using a shade cloth which reduces up to 50% of sunlight, will minimise the chances of plants being affected, especially in the final month. Denser planting also gives protection. Symptoms are more severe on the exposed south and west sides of the beds than on the north and east sides and interiors of the beds. Therefore planting to obtain the least exposure, such as orientation of the beds on a north to south basis rather than an east to west direction is helpful. Nitrogen and magnesium nutrition can affect the high sugar disease. High levels of nitrogen temporarily alleviate the adverse effect of the high sugar, especially when nitrogen is supplied, mainly in the nitrate form rather than in the ammonium form. Magnesium deficiency can accentuate the disease. When high sugar in the plants adversely affects quality, steps should be taken to ensure an adequate magnesium supply. Growers should strive to produce plants that have flat, green flexible leaves free of any excess cupping, thickness, brittleness, crumpled shapes and physiological leaf spot.