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ROOT ROT IN CITRUSTh« gradual and parsiatant decline of citrus tree* in the Mur-mmbidgea irrigation area hae long been of very great concern to citrus grower* in that part, and for a long time the Research officers of the C.8. and I.R. Research 8tation at Griffith have had the matter in hand. Through the efforts of Dr. Lilian Fraser, working in co-operation with Mr. H. J. Braund, Mr. J. V. Hayden and Mr. C. J. Horth, it transpires that the main cause of the trouble is a form of root rot known as Phytophthor* Citrophthora, a parasite flingus which attacks the fibroue roots of citrue trees and causes first of all, dead twig* to appear, the trouble gradually spreads to limbs and eventually kills the tree. The diseased tree* .ire first seen where water logging of the soil occurs and gradually spreads In Its effect as the incidence of water logging spreads. The position in the Mur-rumbldgee area bus been aggravated by a succession of wet winters, and these, following a full summer series of irrigations have created high water tables. This condition ho* been Instrumental in the development of Phytophthora. Another difficulty in the Murrumbidgec area lies In the fact that many of the soils are of Huch a nature as to make drainage a difficulty and the root system Is In consequence rather shallow. Tress examined in various states of decline show that the fibrous roots are mostly to be found in the top six inches of soil, while In bad cases the lower roots are dead. The fungus, Phytophthora Is a particularly virulent one and is capable of destroying fibrous roots In a few days, given favourable conditions such as constant moisture, but In relatively dry soils very little rotting tAkes place and the ney roots are formed quicker than the fungus can destroy them.The virulent nature of the disease has been demonstrated by the inoculation nr the fungus in unaffected trees.As to whence this fungus comes from. It is suggested that there is a strong possibility of »t being a normal constituent of the virgin soils us part and parcel of our soil flora, and when the land is irrigated and plumed with citrus trees the fungus is gradually built up os the effects or over watering become evident.Soils In the Murrumbidgec area nro in tho main, of a heavier type than thoso upon which citrus Is planted in this State and in the citrus areas of Victoria, This makes the la torn 1 penetration of water Blow, and drainage difficult. Tiic condition* thus created stimulate into activity tho latent spores lt;»f Phytophthora. whether this disease occurs In other citrus areas has not yet been determined, but it Is quite likely tlat we will find it. Considering ihat it has also been found in other parts of the world our lower River Murray CHrus nreas are not likely to he free. The fungus concentrate* its energies nn citrus fruilt-, yet some root stocks havo been found tu be almost Immune front attack.Tho most promising root sto k to date is sour orange and TrJfollatn and of these the sour orange hns been extensively used In other parts of the world, but this stork has given conflicting results in the Mnrrum-bldree nren, owing poss My !«* various strains of sour orange stock being used.Some early plantings on this stock have been successful but later planting* have not met with the success of those planted many years ago, Trlfollntn ennnot hi used for lemons and navel oranges, hut ha* given fairly good results with VaU-ndu* and grapefruit. This root Block has a dwarfing tendency on tho trees and is associated with a drop In cropping returns.In the stock trials conducted by tho Division of Horticulture at the Griffith Experimental Station a reduction in crop of one quarter in the case of Valencias, and one third in the case of grapefruit has been indicated. Those estimates are taken from the crop return extending over, ten to twelve years.There Is evidence. wrltee Dr. Fraser, that over-watering alone In the absence of citrus can cause an increase in the amount of fungus in the soil to dangerous levels, so that citrus can only be established with difficulty and shows earlier signs of root rot than those established on virgin land. Instance* of early failure after vineH are known.In one csss investigated a light sandy soil which hed grown only lucerne for mmny year* but which wit probably heavily watarod at times, was found to be infected and citrua could not be established . • . •Phytopthora has been found associated with salt injury. Conditions which favour the accumulation of salt would bo suitable for rapid growth of fungus and it Is possible that In the absence of Pythophthora Injury from salt might not be so severe. . .”In dealing with tho future outlook Ur. Fraser says, When the soil has been badly infested it is permanently unsuitable for citrus and point* out that three alternatives have to be faced;1. To abandon the unsatisfactory task of attempting to grow citrus on soils liable to waterlogging and difficult to drain quickly.2. In future developments to adopt a long term horticultural rotation recognizing the limitations of rough lemon as a stuck on heavy soil. This would require orchards sufficiently large to allow regular planting* to be made to compensate for degeneration.3. To use resistant stock. This seems to offer most promise for the future iv-esfabllshmenf of citrus In the Murrumbidgec Irrigation area but much work on stock selection and improvement Is required.”Owing to the deterioration of many citrus trees in the South Australian Irrigation settlements following a wet winter, tho possibility of trees being affected with phytophthora cannot be overlooked and if the partly defoliated trees do not make a complete recovery In this growing season the matter should be Investigated bv n competent authority. The deterioration of citrus trees has mainly been noticed in the lower portion of .i block which is a place where a high water table is aus-j ice ted. a ml this gives rise to a suspicion that the disease mny be In localities where I he decline of trees has been attributed to salt. Dr. Fraser's research work merits the highest praise she will receive the thanks and appreciation of ull citrus growers.
Newspaper Details
Renmark Murray Pioneer
Renmark, South Australia, AU
Thu, Nov 05, 1942
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USA 21 Jan 2020
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