Vegetative Growth in the Host.—Whether a wound in the host-surface is necessary to enable the fungus to gain entry in the first instance is not known; but, once established, it is an active parasite. Larvae of Sirex juvencus L. have been found in wood infected by Botryodiplodia pinea, and the death of the wood has been attributed to these larvae, the fungus being regarded as merely saprophytic; but, as it is quite common to find branches suffering from fungus attack without there being any trace of the grub, this suggestion is untenable.
As already mentioned, the path of infection can usually be traced back to a node, and then out into the short stems of the cones. It would seem that in some instances cones or cone-stems when young have been the original parts infected, and that the fungus has then worked down into the stem or branch. In other cases, however, infection seems to have taken place directly in the stem or branch involved.
The hyphae, at first hyaline, soon change to a fuliginous shade, and finally become quite black, especially when near the surface of the stem. The hyphae vary in width, but the older, coloured hyphae are usually the largest, attaining an extreme width of more than 10 micromillimetres. Here and there may be seen hyphae bearing on their surface a close arrangement of small papillate outgrowths, suggestive of rudimentary haustoria. These hyphae are intercellular in position, and are usually hyaline. Growth in the cortex is vigorous, and it soon becomes a semi-stromatic mass of crushed dead cells bound together by numerous much-branched hyphae. Later, as the pycnidia are being formed, a true stroma is gradually developed beneath the epidermis, and eventually practically all the host-tissues outside the xylem become a black stromatic mass, which may extend for a considerable distance, parallel to the surface, beneath the seemingly still-intact bark.
The entry of the hyphae into the xylem takes place with few exceptions by way of the rays (text-figs. 1, 2). In radial sections of an infected area almost every cell of the rays, and particularly the median albuminous ones,
will be found to be permeated with the stout, dark strands of the fungus, and throughout the length of the ray numbers of slender branch-hyphae are given off into the tracheids, down whose length they can be traced for long distances. The hypha in a tracheid usually runs straight, but it gives off, at irregular intervals, further branch-hyphae, which as a rule pass directly through the pits in the lateral walls into neighbouring tracheids. Here they in turn commence a straight and usually downward course. Although hyphae in the tracheids are not as a rule as wide as those in the cortex, here and there one of greater width may be seen, almost filling the tracheid containing it. The colour of these larger hyphae, like those in the cortex, deepens with age to a fuliginous shade or to black, eventually causing the wood to appear cinereous to the naked eye.
Fig. 1.—Semi-diagrammatic representation of invasion of the wood by hyphae travelling by way of the rays and thence into the tracheids.
Fig. 2.—Portion of fig. 1 enlarged. The hyphae travel chiefly through the albuminous cells of the rays. × 352.
Reproduction.—When the disease was first noticed, in August of 1924, pycnidia were already present on the surface. The spores in some of them were light brown in colour, but in most they were still hyaline. Similar stages may still be found in the original material as well as in that gathered in mid-November. The production of pycnidia thus continues for some time. The spores are seldom mature when the pycnidia first appear through the ruptured epidermis. Even in pycnidia that are known to be several months old most of the spores are still pale if not actually hyaline. All stages in complexity of pycnidial structure are to be found (text-figs. 3, 4, 5). They may be simple, papillate, and more or less spherical, with the free surface perhaps a little flattened. Two or more may occur in a row or group and have their walls in contact. In the next stage of complexity, in which more pycnidia are usually present, the walls in contact are replaced by common walls, each pycnidium still having its own ostiole; a rudimentary or a well-developed stroma may or may not be present in this stage. In the final stage, which occurs in the largest pycnidial groups, the combined pycnidia are replaced by a stroma with a single locule, one or more ostioles, and one, several, or no partial dissepiments.
The simplest non-stromatic isolated pycnidia are to be found wherever pycnidial formation is only beginning. Later, adjoining these pycnidia, or
in the region close by, stromatic pycnidia and chambered stromata are formed in close juxtaposition. An old row or group of pycnidia exhibits
Figs. 3, 4, 5.—Pycnidia in different stages. In fig. 3 the pycnidium is single and has no stroma. In fig. 4 the pycnidia form part of a small group, and the beginning of a stroma is indicated in the area marked with parallel lines. In fig. 5 the pycnidia are part of a long row. A stroma is present, and one pycnidium is of the stromatic type with a single locule and a partial dissepiment. × 48.
Fig. 6.—Spores, young. × 352.
Fig. 7.—Spores, mature. × 352.
at one time every grade of pycnidial development, the one basal stroma being common to all the pycnidia of the group, with the exception of the
simplest younger ones, which in the ranked arrangement occur usually at either end of the row, and in the group arrangement at the periphery. Even for the youngest isolated members of any group, however, it is usually only a matter of time before they too are linked to the main body of the group by an extension of the common stromatic base. As already mentioned, this stromatic linking has not been found on the cones, where the pycnidia as a rule remain isolated from one another. The smaller size and the isolation of the cone-pycnidia are probably due to the greater hardness of the cones preventing the free ramification of the mycelium evident in the cortex of stem and branch, where, moreover, the hyphae are in direct communication with the water-supply of the xylem.
The size of the pycnidia varies from 250 × 250 to 800 × 800 micro-millimetres. If they are prevented by mutual pressure from expanding, their height exceeds their width. On the other hand, in the complex type with the locular stroma the width is usually increased, and may be several times as great as the height. The ostiole is shortly papillate. The cells composing the walls of the pycnidium are, on the whole, thin-walled, angular to rounded, black on and near the surface, fuliginous in the intermediate region, quickly paling to hyaline in the interior.
The spores (pycnospores) are ellipsoid-obovate to cylindrical; the ends are rounded, the lower one being usually narrower than the upper, at times markedly so. The colour of mature spores is fuliginous to fuscous, but they may be discharged when immature, in which case the colour is often not deeper than a golden-fuliginous. The septation varies. In each row or group of pycnidia there are many in which only the non-septate Sphaeropsis type of spore is to be found, but if the material is kept until some of the pycnidia approach maturity, which may mean for several months, the older pycnidia show at least a small number of uniseptate spores. In certain cases the septate Diplodia type of spore can readily be found, but this is rare except when the material had been kept for a considerable time. As a rule it is unusual to find the septate spore in pycnidia on the cones, where the Sphaeropsis type predominates instead. Spores when mature range in size from 24–37 × 14–18 micromillimetres, the average being about 32–35 × 14–16 mmm. The sporophores are hyaline, and rather stout, attaining a size of from 7–12 × 2.5–3.5 mmm., with an average of 10 × 3.5 mmm.
Nomenclature.—The above description closely approximates that given by Kickx for Diplodia pinea (Desm.) Kickx * occurring on needles of Pinus montana and P. silvestris in France, Belgium, and Italy. Kickx, and Karsten also, reported a form of the same fungus in the bark of P. silvestris and of species of Abies. The only major point of difference between the fungus of Kickx and the present one is that Kickx, and also Desmazière, who originally described it, failed to notice the locular stromatic type of pycnidium occurring on stem and branch when the fructifications are advanced in age. But as the material described was on needles, that in the cortex being only noticed in passing, the significance of the omission is lessened, especially as the stromatic type does not occur regularly in the present disease. Moreover, it is only within the last few years that the stromatic type has been recognized to be a usual phase in the development of the more common Sphaeropsis or Diplodia phases: and, as Desmazière first described his organism as long ago as 1842, the omission of these
[Footnote] * As given in Saccardo, Syll. Fung. vol. 3, p. 359, 1884.
characteristics of the pycnidium is not surprising. Recently von Hoehnel has claimed that a fungus known earlier as Phoma macrosperma Karst. is merely Diplodia pinea (Desm.) Kickx at the stage when its spores are still hyaline. More recently again, Petrak, after insisting on the validity of the genus Botryodiplodia for members with grouped pycnidia and spores over a certain size, gave a detailed description of Phoma macrosperma Karst., adding to it details of structure which had not been given in the original description, and including in particular a greater spore-size than had been given for Diplodia pinea (Desm.) Kickx. This discrepancy he has overlooked, or has perhaps considered it insignificant (seeing that this character may show considerable variation), and, on the strength of his own previous assertions concerning the validity of the genus Botryodiplodia, has transferred Diplodia pinea (Desm.) Kickx to Botryodiplodia pinea (Desm.) Petr. Since then, although the generic limits of the Diplodiae are being more closely examined as knowledge of the developmental variations exhibited by members of the group is acquired, the genus Botryodiplodia as Petrak understands it still stands unchallenged.
The fungus attacking Pinus radiata and P. muricata in New Zealand is therefore referred to Botryodiplodia pinea (Desm.) Petr.