THE CHRYSANTHEMUM

Otago Daily Times, Issue 22398, 20 October 1934, Page 21

THE CHRYSANTHEMUM

EELWORM There is a general impression that some dying off and loss of at least the lowei leaves of the chrysanthemum is to some extent a natural habit of the plant. However, this is not so, and a thoroug.uy healthy plant, properly grciwn, will hold its leaves right to the base of the stem, even through the flowering period, borne varieties of chrysanthemum seem to be very susceptible to eelworm infestation, which often affects the plant so severely that few, if any, flowers deve op. The commercial growing of chrysanthemums is carried out on a large scale in many countries, and recently the writer was privileged in seeing the nursery of Messrs Lowe and Shawyer, at Uxbridge, near London, where there were 1 250,000 pots of chrysanthemums on hand! Some five or six years ago several large nurseries in the Reading district found it had become increasingly difficult to, obtain satisfactory flower crops, especially in certain varieties, despite every care and attention in their culture, so an appeal was made to the Horticultural Department of the University of Reading for some practical help in solving the problem. As a result, Mr W. E. H. Hodson, advisory entomologist at that university, took up the subject, and last year presented a very complete survey of the whole matter. This article is based on his report, and the writer wishes to give full acknowledgment to Mr Hodson, not only for this subject, but for his advice in many other matters which were discussed. It has been known to scientists for many vears that premature death of the basal leaves of the chrysanthemum, along with a serious lack of vitality in the plant, was due to the effects of a particular eelworm, known as the Aphelenchoides Ritzema-bosi. but most growers were either ignorant of the fact or knew of no means of control. SYMPTOMS.

Whenever death of the basal leaves ol a chrysanthemum plant occurs to any extent eelworth infestation should be suspected as the cause. The leaves are the principal source of infection, and it is typical of an attack that patches appear on the leaves at first of a pale green colour, then brownish, and finally black. Eventually the whole leaf dies and hangs dry and black on the stems for some time before falling off. The basal leaves are always first affected, and the trouble tends to spread from them up the plant. This spotting of leaves is characteristic, and distinct from the even yellowing-off which occurs with unbalanced

feeding or faulty watering. A light attack manifests itself during the middle and later portions of a plant’s life. Severe infestation in newly-rooted cuttings may be detected by one or more leaves showing the typical signs, but a milder attack usually does not show until active growth is in progress. In the nearly mature plant, with a serious attack, rapid involvement of the upper leaves occurs, and, as the flower buds develop, there may be a general wilt-ing-off and death of the whole aerial portion of the plant. The worst troubles occur in wet seasons, as moisture is essential for facilitating spread of the eelworm up the plant. DIAGNOSIS. The symptoms and signs detailed are as a rule unmistakable, but it is easy to demonstrate the eehvorms by breaking up an apparently infested leaf on a watchglass and adding a little water. Soak for 10 minutes, remove the broken leaves, and examine under a microscope or with a very powerful lens, with a magnification of not less than Xls. If numerous minute eehvorms can be detected in the water the diagnosis for all practical purnoses is completed. THE EELWORM ITSELF. The actual worms are very small, being usually under l-25th of an inch in length and very thin. The complete development from egg to adult takes only about 10 days, and, under favourable circumstances, a generation from egg to egg can be completed in 14 days. The worms live actually in the tissues of the leaves, buds, and outer layers of the stems. They wander on the outside of the plants in films of moisture, entering the tissues by way of the stomata or pores. The parent, chrysanthemum stools remain infected from fallen leaves, and the worms are capable of living in the soil for several months. Thus, cuttings taken from infested stools spread further infection. In the dried leaves adult worms become curled up, and can remain viable for three years, needing only suitable moisture to become active. It is to be noted that the worms travel on the surface of the leaves and stems, and ascend the growing plant when moisture is present, and are not carried up by the growing portions of the plant. This explains why the lowest leaves are the first affected. .Finally, when the plant is out back at the end of the season, the stool remains infected by worms, which have migrated to it direct or from leaves which have fallen to the ground. Consequently, any cuttings taken from an infected stool must necessarily carry further infection on the young plants. Also, any soil used from an infected bed or plants put back into infected soil act as further means not oulv of perpetuating, but of spreading, the nests. CONTROL MEASURES. From what has been said, it is apparent that eradication of the pest, once established, is very difficult. Since migration of the eelworm can only take place in the presence of moisture, dry conditions are unfavourable. For similar reasons, light, dry soils are less favourable to the pest than heavy, wet ones. As far as is known at present, eel worms in general can only be eradicated from soil by starvation or by heat. Also, within plant tissues they can only be killed by heat, carefully regulated, so as not to destroy the plant. The chrysanthemum eelworm travels from leaf to leaf on the plant in the presence of moisture. These facts give us indications for possible means of control: (a) Soil Treatment. —Steam sterilisation is the ideal, but, in most cases, is impracticable. Chemical means have so far proved ineffective, but iodine treatment, as suggested by the writer, opens up possibilities, at any rate, for rmali areas. It is still in the experimental stage, but further investigations are being conducted in England, as well as here. Watering the stools of infested plants with a solution of iodine in water, I to 2000, a day or two prior to taking cuttings, and then rinsing them in a 1 to 4000 solution before planting, is suggested as a means df control. Of course, planting must be done in soil free of eelworms. Infested beds should be rested for two years if possible, or utilised for plants that will not act as hosts for the para(b) Spraying. —Hodson states that regular spraying at 14-day intervals with a strong nicotine and soft-soap wash

(nicotine lOoz to soft soap 81b, or more, in hard water, and water to 40 gallons) does give an effective control of eelwom, but will not eradicate -it. Other observers report favourable results with a 3 per cent, ammonia solution. (c) Hot water treatment. It was found that if unrooted cuttings are subject to sufficient heat to kill eel-worms a large proportion of plants die. For obvious reasons, such as treatment of newlyrooted plants is impracticable, so Hodson experimented with the stools, and has evolved a successful method. It is certainly less costly than nicotine spraying/ and gives quite healthy young plants, although there is a definite' retardation of shoot production. The stools when lifted should have as much loose earth knocked away as is possible, and as much of the remainder washed off as can be done. Then place in a hot water bath at a constant temperature . of 110 degrees F. for 20 minutes if small or 30 minutes if the stool is of moderate or large size. On removal, cool off at once in clean water, drain and box up, or heel in the stools in dry earth, away from any infected areas. Every effort must be taken to avoid recontamination from outside sources. Treated stools may be expected to produce shoots for cuttings some two or three weeks later than untreated onesi With correct treatment and reasonable care of the stools, Hodson states that the mortality should be nil, and the resultant plants entirely free oi eelworm. , ~ , , (d) Ringing of plants. It has been recommended to put a half-inch ring of birdlime or tanglefoot round the stems of chrysanthemum plants, renewing it as the stem grows thicker. Plants must be so placed that their lower leaves do not touch the soil or come in contact with other possibly infected leaves. It seems definite that this is a practicable means of checking the spread of disease in, a growing plant. , . , . . ~.. It would be helpful to all interested if readers would place on record their experiences with the treatments suggested, and especially as regards the iodine method. HOST PLANTS OF APHELENCHUS RITZEMABOSI. It is unfortunate that a number of garden plants are liable to attack by this pest, as it is essential in trying to clear the soil of it that host plants must not be planted in chrysanthemum beds, and vice versa. Of the commoner garden plants the following act as hosts of this eelworm—viz.; Delphinium, Rudbeckie. Dahlia, Verbena venosa, Michaelmas ' Daisy, Asters, . Pentstemon, Phlox drummondi, and Zinnia. Others listed are Calceolaria and Gloxinia also Ranunculus repens (Creeping Buttercup), Senecio vulgaris (common Groundsel), Adenostyles, Doronicum, Oncium, Elstolyia cristata and Valeriana montana. In making the iodine solution, it .is necessary to mix potassium ipdide with the iodine crystals and grind in a pestle and mortar. The mixture dissolves readily in water, whereas iodine alone is very insoluble. For small users, any chemist would make up a concentrated solution, and give directions for necessary dilution. —By Dr Hugh R. G. Poate, F.R.H.S., president R.H.S. of N.S.W., in the. Australian Garden Lover.