46 



THE AGRICULTURAL NEWS. 



February 4, 1911. 



FUNGUS NOTES. 



TWO DISEASES OF CITRUS TREES 

 IN FLORIDA. 



Of recent years a disea.se known as scaly bark has 

 caused considerable damage to sweet orange trees in Florida, 

 and has been in consequence the subject of careful study 

 extending over the last few years. Professor Fawcett, Plant 

 Pathologist on the StafT of the Florida Experiment Station, 

 has recently published a full account of the disease, its 

 cause and treatment, in the Annual Report of the Florida 

 State Ej'periiiiirit Station for 1910. A second disease known 

 as scab or verrucosis is also described in the same article, by 

 this author. The two diseases will be considered below. 



SCALY b.\i;k. The disease occurs principally on sweet 

 orange trees, but it may be found on old trees of ihe 

 grape-fruit and lemon. It appears first most commonly, on 

 twigs varying in age from nine to eighteen months; it may, 

 however, conmience its attack on much older branches, 

 though it is very rarely found on twig.s less than six months 

 old. The attacks most generally commence between June 

 and December, a period which corresponds closely to the 

 rainy seasons in Florida. The first noticeable stage of the 

 disease consists of the apjiearance, on the epidermis of the 

 branch attacked, of circular or oval si)ots from 1 to 4 mm. in 

 diameter. The spots consist in .some cases of a slightly 

 raised ring, composed of small pustules appearing like breaks 

 in the epidermis; in other cases the spots commence as lemon- 

 coloured areas of api)roximateIy the same size as in the 

 mature condition. The bark of infected areas turns 

 rusty in colour, so that at the end of eight or ten months 

 the spots are of this colour, have a well defined margin, 

 and are from 10 to 20 mm. in diameter. Later the 

 bark becomes brittle, cracks, and forms small flakes. The 

 spots are at first separate, but increase in number and become 

 joined together, and finally, at the end of two or more years, 

 the branch is ringed, and dies. These symptoms are frequently 

 accompanied by exudations of gum. On older liranches 

 and main stems the disease is rarely fatal, and merely 

 causes a rough irregular appearance of the bark. Even on the 

 smaller brandies the action of the disease is slow, though in 

 many cases it is hastened liy sul:)sequent infection by spores 

 of Colletotrirlium ffloeosporioideS:,the witlicr-tip fungus of citrus 

 plants. The .spores germinate and grow on the diseased spots, 

 and the resulting mycelium secretes a poison which causes the 

 death of the branches. 



The disease can also attack the fruit, on which it usually 

 Commences to appear in the months of July and August, when 

 the green fruits are about i-inch in diumeter. The spots 

 occur on the rind only. They are from 2 to 3 mm. in diameter 

 at first, but extend to a diameter of 5 or 10 mm. They occur 

 most frequently in the form of rings or bands similar to those 

 on small branches; they may also commence as round, yellow- 

 isli areas. Later, the rings become sunken and brown, while 

 the central portion remains green. Eventually, the fruits turn 

 yellow prematurely, the spots .Ijecoijip brown throughout, and 

 the fruits drop. ' ' 



It has been shown by means of carefully conducted 

 infection experiments that the diS(^ase is duo to a fungus, 

 Ilorriiodendron sp., whose mycelium a'td spores occur partly 

 on the surface of the di.seased areas. .**'■' 



On different culture media ffte'fi^^us forms a dark -green 

 or black mycelium, consisting of septate hyphae, with strong- 

 ly marked constrictions at the septa. The si)ores are liorne 

 on upright conidiophores. They ■kre'hiore or less circular. 



dark in colour, and occur in chains or branched chains. 

 Branches arise from any segment near the end of the conidi- 

 cphore, and also give rise to chains of spores. 



Four lines of treatment have been recommended for this 

 disease. The first consists of top-working infected trees to 

 immune varieties of citrus. In the second, the tops of the 

 trees are cut off", so that only the trunks and basal portions 

 of the main branches are left. The trees are then washed 

 with a mixture of equal parts of carbolineum and water in 

 which soap has been dissolved, the mixture being carefully 

 painted all over the bark. This is done about the month of 

 February. At the end of the following growing .season 

 a strong, healthy growth may be expected. In the third case, 

 the trees are carefully pruned, and sprayed with a l-to-5-per 

 cent, emulsion of carbolineum in soap and water. E.xperi- 

 ments on the effect of spraying with this solution have, how- 

 ever, not [irogressed sufficiently to give any very definite 

 results. The last line of treatment consists in .spraying the 

 trees for two or three years with Bordeaux mixture, made up 

 on the 5-5-50 formula. Spraying should be carried out three 

 times each year — once before the flowers open, once when the 

 fruit has set, and once when it is about half-grown. In 

 Florida, it was found that this treatment has an injurious 

 effect on the fungoid parasites of the scale insects, so that 

 these .should be reintroduced into the trees after the final 

 spraying in each year, to prevent damage by the insects. 

 If this point is attended to, .spraying proves successful. It 

 would appear, from the results of the experiments in these 

 various lines of treatment, that at present, lioading back and 

 treating with carbolineum, and spraying with Bordeaux mix- 

 ture, are the most successful methods. 



SCAB. This disease attacks lemons, sour oranges, satsumas 

 and grape-fruit, in Florida. It is rarely found on sweet orange 

 trees. 



The attack commences on young leaves in the form of 

 light-brown or cork coloured spots. These become depressed 

 on one side and raised on the other. They are of a dark- 

 brown or sometimes pinkish colour. The separate spots 

 coalesce until an irregular corky .scab is formed, while the 

 leaves become twisted and contorted. The fungus may also 

 occur on the fruit, which then presents a warty appearance. 



The di.sease has been pnived by inoculation experiments 

 to be due to a fungus, Cladosporivm ritri. It can be pre- 

 vented by the use of Bordeaux mixture. This should be 

 sprayed on to the trees early in the year, before growth com- 

 mences. A second applicatioji may be made later if the 

 disease appears on the young fruits. As is stated al>ove, care 

 must be taken, when using this mixture, that a watch is kept 

 on the scale insects, and if necessary, that measures are 

 employed to prevent their undue increase. 



Ill connexion with the two diseases described above, 

 matter of some interest is furnished by Mr. C. K. Bancroft 

 in a papey, published in the Ayinah of Botan;/, Vol. XXIV, 

 No. XCIV, April 1910. The author found that a large number 

 of different host plants were subject to a leaf disei^se due to 

 a .species of Hormodendron. This fungus he found to be 

 identical ;with Cladosjiorium /terl/(iriim, a common sapro- 

 phyte. His investigations further led to the conclusion that 

 the Hormodendron form ^\as parasitic, and occurred in tlie 

 summer; while the Cladosporium form was a saprophyte, and 

 occurred in the winter. At a low temperature, the last- 

 mentioned form reproduces itself, while at a high tempera- 

 ture it gives rise to the Hormodendron spores. It would be 

 interesting to discover if any parallel to this is furnished by 

 the scaly' bark and scab fungi of Florida. 



