MYCOLOGY AND PLANT PATHOLOGY Virus Diseases of Plants .sx Rothamsted Experimental Station .sx By the use of collodion membranes of known and graduated pore size , it has been found possible to determine , on the one hand , the size or pore through which a particular virus can pass , and on the other , the size which prevents it from passing .sx The pore sizes differ with different viruses , e.g. the virus of tobacco Mosaic just passes pores of 0.051 Aucuba Mosaic virus of tomato passes 0.112 but not 0.1 , a Hyoscyamus Mosaic virus passes 0.30 but not 0.23 .sx From these data it can be deduced that the sizes of the viruses , or perhaps of particles to which the viruses are attached , vary from 15 in tobacco Mosaic to 150 for the Hyoscyamus virus .sx The method allows , in some cases , of the separation of two viruses occurring mixed in nature .sx A rapid and convenient method of determining the quantity of virus contained in a sample of plant juice has been developed by the use of a necrotic-spot technique , the number of spots varying directly .sx with the concentration of virus .sx This is further evidence in support of the view that virus exists in a particular state , and not diffused throughout the liquid .sx For the first time a method has been discovered of producing in the living non-virus plant intracellular inclusions which are indistinguishable from those produced by virus .sx This consists in supplying the plant with a salt of molybdenum , and up to the present no other substance has been found to give the same results .sx The artificial lengthening of the day during the winter months , referred to in last year's Report ( p. 49 ) , continues to prove thoroughly satisfactory .sx Experimental and Research Station , Cheshunt .sx During 1932 tomato plants suffering from Spotted Wilt were received for the first time .sx They came from widely separated localities in Great Britain .sx The disease was studied and found to be identical with the Spotted Wilt of Australia , but distinct from the Mosaic and Stripe diseases of tomato of this country .sx Clean cultural methods and the eradication of Thrips are emphasised as control measures .sx There is a danger of the disease over-wintering in Dahlias and Chrysanthemums .sx Experiments with treatments calculated to improve infected plants have not given conclusive results , but some success has been obtained in nurseries by methods which increase the rate of vegetative growth .sx Two outbreaks in Cinerarias ( Kent and Herts ) have been examined .sx Investigation of tomato diseases of the Mosaic , Stripe and Streak group has been continued .sx Ordinary or mild tomato Mosaic has been identified with true tobacco Mosaic and found not to be implicated in Stripe .sx The symptom-picture in the tomato which characterizes Stripe may be due to several causes .sx As the diseases are usually indistinguishable on tomato the term Stripe will doubtless be retained by growers , but the following more precise nomenclature is proposed :sx the name stripe is suggested for use when the disease is attributed to Bacillus Lathyri ; Glasshouse Streak when due to a single virus ; while Streak , in conformity with American terminology , is reserved for a mixed virus infection ( typically tomato Mosaic and potato Mosaic) .sx The viruses involved have been described .sx It is of practical importance to know the type of infection in a given case , because , although Stripe and Glasshouse Streak respond to treatment with potash , Streak does not .sx Applications of nitrogenous compounds benefit plants affected with Streak .sx Investigations of the effect of the virus of Aucuba Mosaic of tomato on the metabolism of affected plants from the time of inoculation up to the appearance of severe mottling have been continued .sx When healthy and also inoculated plants are examined immediately before and after 12 hours darkness , the amount of starch in the diseased plants is slightly greater than in the healthy plants .sx This holds good until the appearance of severe symptoms , when the amount of starch in the diseased plants is much less than in the healthy plants .sx The amount of reducing and non-reducing sugars in the diseased plants usually decreases during the night , whereas that in healthy plants increases .sx The total amounts of starch and sugar in the leaf ( lamina and petiole ) of diseased and healthy plants , sampled at three hourly periods throughout 24 hours , show marked differences ; and , though several theories can be suggested to account for this , more work is necessary before a definite conclusion can be reached .sx In view of the greater prevalence and severity of Mosaic disease of tomatoes in certain parts of one and the same house , investigations are in progress to determine the relation between soil conditions and the incidence and extent of tomato Mosaic .sx Scottish Society for Research in Plant Breeding , Corstophine .sx The investigations into the carbohydrate metabolism of potatoes affected with virus diseases has been continued .sx Those on the carbohydrate metabolism of Leaf-Roll and healthy potatoes were completed in 1931 and the results published .sx That on the carbohydrate metabolism of Crinkle-infected plants was completed in 1932 , and the results will be published shortly .sx At the same time , the nitrogen side of metabolism has been studied and the work on healthy and Leaf-Roll potatoes has been completed .sx The results will shortly be available for publication .sx In brief it has been found that nitrogen metabolism in Leaf-Roll plants does not markedly differ from that in healthy ones , and Leaf-Roll is not a disease in which the nitrogen metabolism is distributed , as has hitherto been supposed .sx The work at the Ainville Sub-Station consisted in the growing of plants in insect-proof greenhouses for seasonal analyses in the nitrogen work on Leaf-Roll .sx Work at the Huntly Sub-Station has again been confined to the Mosaic diseases of potatoes .sx In 1932 an investigation was carried out on the carbohydrate metabolism of healthy potatoes and potatoes infected with the X and Y viruses of Smith and Salaman .sx The analytical side of this work has been completed ; the data are being statistically examined and will be published in due course .sx For the genetical side of the work , it has now been decided to carry out the actual crossings between the various varieties and also between healthy and diseased plants at the Corstophine Station , and a programme has been arranged for 1933 .sx Resistance tests with possible commercial varieties having a high resistance to or tolerance of virus diseases will also be carried out at Corstophine .sx Seedling progenies will now be grown entirely at Huntly , and a start is being made with seed obtained from various crosses made at Corstophine in 1932 .sx Cotton Disease Research .sx The work at the Rothamsted Experimental Station on the Angular Leaf-Spot or Blackarm disease of cotton , which has been assisted for four years by the Board , has been brought to a conclusion on the departure of Dr. Stoughton from Rothamsted .sx The work on the influence of environmental conditions on the disease , which has been carried out in the special control chambers erected under a grant from the Empire Marketing Board , has led to a number of definite conclusions .sx The results of six years' work on this aspect of the disease may be briefly summarized :sx Primary infection of the cotyledon is usually due to bacteria carried on the outside of the seed and in the fuzz .sx Thorough disinfection of the exterior of the seed results in healthy seedlings .sx Soil temperature affects the amount of primary infection , a constant soil temperature above 30 C. reducing the degree of .sx infection , but infection is not inhibited at 40 C. A regular diurnal variation in soil temperature produces the same effect on infection as a constant temperature near to the mean of the fluctuations .sx Soil temperature is of importance as regards the disease only during the first two or three days after sowing .sx A high soil moisture content favours the primary infection .sx Primary infection is affected by the type of soil in which the plants are grown .sx Soil temperature has little or no effect on secondary infection resulting from spray inoculation of the plants .sx Plants diseased in the seedling stage grow out free from disease if no further inoculation occurs .sx Air temperature has a marked effect on secondary infection .sx Maximum infection occurs at a constant air temperature of 35-38 C. , with decreasing incidence at progressively lower temperature .sx A regular diurnal variation in air temperature exerts an effect on the amount of disease equivalent to that of a constant near to the mean of the fluctuations .sx Experiments on abrupt changes of temperature show that air temperature is of importance throughout the incubation period of the disease ; the actual temperature at the time of inoculation is comparatively unimportant , except in so far as it affects the mean temperature during the incubation period .sx The amount of infection at a given temperature , or mean equivalent temperature , is conditioned by the atmospheric humidity at the time of inoculation .sx High humidities favour infection , low humidities reduce it .sx Humidity is of importance only during the first two days after inoculation .sx PLANT BREEDING .sx Welsh Plant Breeding Station , Aberystwyth .sx In the spring of 1932 the seed of seven pedigree strains of grasses were released to the trade for multiplication .sx These included one strain of cocksfoot , four of timothy , one of rye-grass , and one of red fescue .sx Forty acres of pedigree grasses and clover were harvested during the year 1932 , and eighteen additional acres were sown down .sx In all , the seed from thirty strains , eighteen of grasses and twelve of clovers , were grown in ten counties by thirty-three growers .sx The main bulk of the seed was produced in two districts , namely , the Welshpool .sx district and in the Wye Valley .sx The first multiplication of new red clover strains on the other hand was chiefly conducted in non-arable districts , such as the Towy Valley .sx Harvesting conditions were favourable on the whole , and the quality of the seed appears to be as high as any imported from the best seed growing countries .sx Two pedigree strains of red clover were carried to the second multiplication stage by the sowing out of three acres , which will be harvested for seed in 1933 .sx Further progress has been made with a view to the better under-standing of the proper methods for growing remunerative seed crops on a field scale as applied to different types of pedigree grasses .sx Two aims have been kept constantly in view .sx to increase the yield of seed and .sx to reduce the cost of production .sx Problems peculiar to indigenous grasses in connection with the dressing of their seeds have been investigated and appropriate machinery has been designed to overcome various difficulties .sx Preliminary studies have been initiated by resort to a new technique with a view to measuring the efficiency of distance isolation for seed production in the case of both cross-fertile grasses and clovers .sx The work on delayed establishment in some species of grasses is being continued and important results from these investigations have been published .sx Arising out of this work an investigation has been commenced into the relations between the mineral nutrition of grass-seedlings and their reaction to diminished illumination .sx A detailed study has been made of the exact manner in which soaking the " seeds " of cocksfoot in water causes an acceleration of germination .sx It is hoped to publish this work in the immediate future .sx Further experiments are being conducted on the germination of grass " seeds " in solutions of cane sugar ( " suction-force " determinations ) but little material is available that is suitable for this purpose .sx The work on peaty hill soils is continuing ; it is probable , however , that another season's growth will be required ( making three years' growth in all ) before a definite account of the results can be provided .sx The investigations which have been in progress for some time on occurrence of buried weed seeds in the soil underlying pastures of various types have been continued and a report on the first batch of fields investigated will be published at an early date .sx Last year's report referred to the visit to Australia of Mr. William Davies , the Empire Grassland Officer , at the invitation of the Common-wealth Council for Scientific and Industrial Research and the Australian Dairy Council .sx Mr. Davies returned to England in December after a year's stay in Australia .sx