Probably the most serious disease of Viola and Pansy is that known to thegrower as root and stem rot. It is really a disease complex caused by more than one organism but generally by fungi rather than animals. The leaves of typically diseased plants show the first visible signs of attack by the yellow colour of the margin. This colour spreads over the leaf and is followed by tints of purple or purple-brown. Finally the leaves wilt and shrivel and the whole plant collapses. In certain attacked plants a sudden wilting without colour changes in the leaves may occur. When the disease is due to fungi it is usual to find the whole root, system, and frequently the lower part, of the stem, discoloured and decayed. Three kinds of fungi normally cause this decay and each has a fairly typicaI effect on the tissues in which it lives. With a little experience it is possible to separate diseased plants into two broad classes.
This fungus is normally confined to the stem just a little above and below soil level. The area attacked quickly becomes dry and brittle and bears on its surface abundant black or greenish black sporing pustules of the fungus. The parasitism of this fungus has been demonstrated by Mr. N. C. Preston (The Parasitism of Myrothecium roridum Tode) in Trans. British Mycological Society, Vol. 20, pp 42-251, 1936).
These fungi attack both root system and stem and may travel up the latter for a considerable distance above soil level. Sometimes the infected tissue is dry and brittle and then Corticium Solani mycelium is usually present alone in the diseased area. If the infected tissue is soft and moist species of Pythium are almost constantly present but mycelium of Corticium may occur as well. It is this class of plants that we have studied.
The disease is usually of epidemic severity during the period following the first set of blooms but a slow insidious form of the disease occurs on cuttings or second year plants during the winter months. In this latter form of attack two or more organisms may be present in the tissues, as is indicated in the table below, which represents the examination of a collection made from one nursery in March, 1935.
|Number of plants attacked by|
|Variety and no of plants examined||Pythium sp.||Corticium Solani|
|Queen's Park Fancy||(4)||2||1|
Usually only one fungus is responsible for an epidemic attack. This - Summer - aspect of the disease may be ilustrated by a collection of plants taken from a series of beds planted with cuttings in March 1934, and examined after flowering, on 14th June, 1934. Pythium species were observed in not less than four plants of the following varieties: 'John Adamson', 'Jas. M. White', 'Essie Johnson', 'Grendawalda', 'Milton Jumbo', 'And. Jameson', 'Amy Johnson', 'E. Goddard' and 'J. Bastock'. In June 1932, four plants of 'Mauve Seedling', two of 'H. J. Milner' and 10 of an unnamed yellow variety were received from a Midland grower and were found to be infected with Corticium Solani. While not within the scope of our work, it may be worth while mentioning that it is during the same period (April to July) that the severe epidemics due to Myrothecium occur. These have been noted each year from 1932 to 1939. Pythium species have been recorded, in the monthly summaries of Plant Diseases issued by the Ministry of Agriculture and Fisheries, as associated with this type of disease in many parts of England. In 1926 Solberg reported the presence of Pythium de Baryanum and other Pythium species in diseased roots of cultivated species of Viola in Norway, and in 1934 Kaven described Pythium de Baryanum as a common cause of 'Blackleg' or root-rot of Viola in Germany. We have observed species of Pythium similar to those described by Solberg (including a species with spiny oogonia (Fig 1A (A1-5) and one with small oospores (Fig 1A (B1-4)), in several varieties but the most common species is Pythium type D. It has been identified in the tissues of the following varieties:-
|John Adamson||Jessie||William Neal|
|Arkwrights Ruby||Essie Johnson||Pickering Blue|
|J. Bastock||Amy Johnson||Mrs Plumer|
|Margaret Bishop||And. Jameson||Queen's Park Fancy|
|Blue Stone||Milton Jumbo||Sue Stevenson<|
|Blue King||Jas. Leslie||Seagull|
|Charney||Chris. Littlejohn||Councillor Steel|
|E. Goddard||Lavender Profusion||White Swan|
|Greenwalda||Harry Merryfield||Peggy Trail|
|White Gracilis||Maggie Mott||Jas. M. White|
|Mary Hatton||Lord Nelson||Wm. White|
Species of Pythium were first isolated in July, 1934, and since then have been frequently obtained from diseased material. When compared with one another it was evident that the three species existed which exhibited certain constant differences in appearance and growth characters. These species have been called types D, E and H. D is the commonest and has been constantly isolated from infected tissue. E was obtained once from some hybrid Violas sent for examination from Wales and H was isolated from soil surrounding certain infected plants. When the three species are grown upon agar jelly, H exhibits more profuse growth than D or E, while the mycelium of E is definitely more delicate than that of D or H. The oogonia (Fig 1.B) in each species have from one to several antheridia. There are slight differences in the average diameter of the oospores (or resting spores) (Fig 1.C) developed upon maize extract agar (D-21, E-20, H-19.5 microns). Conidia (asexual spores) (Fig 1.D) of E are very irregular (28 x 23 microns) and have seldom been observed to germinate. Those of D (Fig 1.E) and H (Fig 1.H) are more nearly spherical (D-24 x 23; H-24 x 22 microns) and constantly germinate by means of a germ tube (Fig 1.E and F).
On comparing the rates of growth of the three species at seven different temperatures it will be seen from the table below that H has a wide temperature range with a high optimum point whereas D and E have a more restricted range and a lower optimum.
|Temperature of incubation in ºF.||Diameter in millimetres after 36 hours|
The three species grow well on a variety of media, but on stable manure H grows fairly actively, whereas E makes little or no growth and D is intermediate in character. Growth on oat chaff is similar but more vigorous. In garden soil and leaf mould only H shows any growth.
When placed in soil immediately below the roots or round the collar of established plants the three species of Pythium are each capable of causing typical root and stem-rot. This is also the case when the species are grown upon sterile vegetable matter that is later mixed with garden soil which, after a resting period of five days, is planted with clean Viola seedlings. These experiments are merely preliminary to more extensive Summer trials but they indicate that the species are definitely pathogenic.
During the investigation of this disease particular attention has been paid to the character of the soil from which diseased plants have been obtained and it has been observed that the disease is most abundant upon heavy clay soils or soils rendered heavy by the addition of large amounts of organic matter. In almost every case the soil has been exceedingly moist. The type of root growth made by Viola plants in open, well-drained soil is very different from that in heavy, rather moist soil. In the first type the root system is highly developed and penetrates downwards and on all sides from the original ball of soil removed from the frame with the young set. In the second case the root system is mainly confined within the original soil ball and the individual roots are thick and lack the extensive branching of those in open soil. It is the root system of the latter type which, in our experience, suffers easily from disease. An example of this effect is shown in Fig 2. which Illustrates the root systems of three plants which were collected recently. Each plant had been inserted as a cutting in the Spring of 1934. A has grown in open, fertile soil with good bottom drainage, B and C have grown in heavy soil which was mostly water-logged during the Summer of 1934. The root systems differ not only in extent but also in texture, and those of B and C are typical of systems prone to root-rot, which is actually present in the case of C.
We have commenced experimental work upon the soil properties most suitable for Viola growing. While the results so far obtained cannot be regarded as conclusive there is definite evidence of better root growth in soil containing ordinary peat than in soil containing either leaf mould or manure. We consider this is due to the fact that the peat is evenly dispersed throughout the soil thus giving a reasonable moisture content without local clogging, whereas with certain leaf moulds and especially with stable-manure the soil must be very carefully worked before a similar dispersion of the humus particles is obtained. Experience during the Viola trials at Harborne Park, and while visiting growers throughout the country, shows that on heavily manured ground outbreaks of disease are more frequent than upon lighter, more open ground. From field observations it seems to us more likely that stable manure acts physically in producing local water-logging and hence unhealthy root action, especially on somewhat heavy soils, than as a suitable culture medium for the Pythium. This is in some measure confirmed by the fact that the Pythium does not grow freely when cultured upon manure.
Root and stem rot should yield to adequate cultural treatment. Given fertile open soil, which promotes deep action and gives good ventilation, along with the protection of the ground surface from undue heating, the Viola should prove quite hardy. If the Pythium is present in any soil, as it is quite likely to be, a check of root growth will give a better chance of attacking the plant. Prompt removal and destruction of any infected plants along with the use of Cheshunt Compound watered along the rows between the plants and not over the individuals has proved effective in preventing the spread of infection.
Sterilisation of the soil used in striking cuttings or raising seed will remove the sources of infection, but there seems no reason to suppose that the use of fresh, open soil and sharp, clean sand will not give as good results if the cuttings are all of good quality. The disease can only be carried by cuttings if these contain infected tissue and are taken from unhealthy plants showing soft, discoloured stems, but a cutting, though clean, may be of poor quality and produce a few feeble roots which are easily infected. The soil into which Viola plants are transferred for bedding will give healthier plants if it is not over rich, and the grower will do well to remember that the large sappy plant which throws oversize blooms is less resistant than the smaller, more shrubby plant.
We wish to record our deep gratitude to the very numerous growers who have helped with material and permission to visit their gardens to make this investigation possible, and we wish, particularly, to record our thanks for the help of the late Mr Close, who spent much time and energy in collecting material for us.
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