Contamination of seed potato tubers grown from stem cutting or microplant-derived tubers by Phoma foveata and Polyscytalum pustulans in Scotland: the influence of site and fungicide treatment at harvest

A survey was made over 5 years of the incidence of contamination by Phoma foveata and Polyscytalum pustulans on 3-year-old virus-tested stem cutting (VTSC) stocks assessed on three occasions during the storage season, and on their daughter tubers at five farms. The contamination occurring on plants grown in small plots from stem-cutting/microplant tubers was also assessed at the farms.
Fungicide treatment soon after harvest reduced the incidence of both pathogens, especially P. pustulans . Tuber contamination was similar for samples collected after fungicide treatment and before planting. The incidence of contamination on daughter tubers, relative to that on the untreated mother tubers, was less with P. foveata but was similar or greater with P. pustulans . Contamination by both pathogens varied between farms and was reflected in the degree of contamination occurring on daughter tubers from stem-cutting/microplant tubers.
The incidence of P. foveata on stems, soil and tubers of seed stocks classified at Foundation Stock grade from various regions of Scotland was assessed over 2 years. The incidence of tuber contamination was correlated ( P < 0.05) with the amount of tuber-borne soil contamination for all comparisons, but was correlated with stem inoculum in only 7 out of 13 comparisons. The largest amounts of stem inoculum of P. foveata were found in stocks in Aberdeenshire.
These results are discussed in relation to the spread of both pathogens to healthy VTSC stocks.     

The effect of time of planting inoculated tubers on the incidence of potato blackleg and gangrene

In a series of trials in the years of 1973-76 inclusive, chitted seed tubers of potato cvs Ulster Sceptre, Majestic and Pentland Crown were immersed immediately before planting, on three dates in each year, in water, suspensions of Erwinia carotovora var. atroseptica, Phoma exigua var. foveata or a mixture of both. The development of blackleg was greatest in cv. Ulster Sceptre and most rapid after late planting. Its final incidence was not closely related to date of planting except in cv. Pentland Crown which was least affected when planted late.
Combined inoculum of P. exigua var. foveata and E. carotovora var. atroseptica increased five-fold the number of plants that failed to emerge and often retarded early growth of the remainder. Failure to emerge was more frequent the earlier seed tubers were planted. Yield was affected most by blanking and blackleg in the Erwinia- inoculated plots and was also reduced by the pre-plant dip in a P. exigua var. foveata suspension, especially after early planting.
Seed dipped in the mixed suspensions yielded progeny that after wounding and cool storage developed a lower incidence of gangrene than progeny from seed dipped in P. exigua var. foveata alone. There was no evidence that planting time influenced the incidence of gangrene during storage.     

Dry rot of potato [Fusarium caeruleum (Lib.) Sacc.]. Investigation on the sources and time of infection

Inoculation with soil samples proved that the fungus causing dry rot is frequently present in field soils in Cheshire and in soil adhering to imported seed tubers. The fungus was viable in soils having a wide range of p H values and in fields which had not grown potatoes for 5-6 years. Other sources of infection include lofts, used sacks, seed boxes, diseased tubers, and knives used for cutting seed potatoes.
Bruised tubers stored in heavily contaminated boxes developed much dry rot; far less disease occurred in unbruised tubers. In boxes containing own-saved seed, healthy tubers in contact with diseased ones remained sound. Bruised tubers in contact with, or contaminated by, diseased tubers contracted dry rot. Cutting seed with a contaminated knife increased the disease seven-fold.
Inoculation of tubers attached to the parent plant showed that little or no infection occurred before lifting. In field trials severe dry rot developed in several varieties 6-8 weeks after lifting.
The results are discussed in relation to seed treatment.     

The effect on gangrene incidence of dusting seed potatoes with benomyl at time of planting

In 1 out of 2 years' field trials benomyl applied as a dust treatment at time of planting seed potatoes resulted in an increase in potato gangrene in the progeny of both gangrene-free tubers planted in land contaminated with Phoma exigua var. foveata and gangrene-diseased tubers planted in clean land. Gangrene-infected seed tubers treated with benomyl also produced more stems infected with var. foveata than untreated tubers. Two hypotheses are presented to account for this increase in gangrene which does not occur in seed potatoes treated shortly after lifting. In all cultivars tested an organo-mercury dip-treatment increased total numbers of tubers in the seed and chat-size grades without increasing total weight whereas benomyl dust increased the numbers in these grades in Majestic only. The treatment of gangrene-diseased seed with benomyl dust affected neither total weight nor total number of tubers.     

Treatment of seed and ware potato tubers with thiabendazole for control of storage diseases

In March 1977 and 1978 King Edward seed potatoes from three commercial stocks and one stock derived from stem cuttings (healthier seed) were treated with thiabendazole at 320 g a.i./t and 46% of the material applied was deposited on tubers. Seed was stored on trays to sprout and in April samples were planted in replicated plots on two farms in Lincolnshire. Treatment did not consistently affect numbers of stemslplant or total yields but decreased infection of underground stems by Polyscytalum pustulans and Rhizoctonia solani and of tubers at harvest by P. pustulans, R. solani and Helminthosporium solani. In samples stored for 20 wk at Sutton Bridge Experimental Station seed treatment decreased the incidence of skin spot from 25 to 4%, of black scurf from 36 to 20% but had less effect on silver scurf (untreated 52% tubers, treated 47%) and did not affect the incidence of black dot (Colletotrichum coccodes) on stored tubers. In another series of experiments 1 t samples of seed from six King Edward stocks (1977) and four King Edward and one Maris Piper stocks (1978), collected from different farms in Lincolnshire in March, were treated with thiabendazole at 40 g a.i./t. Tubers retained 48% of the material applied. The treated seed was planted on the respective farms in a single large plot adjacent to untreated seed similarly stored and sprouted. Seed treatment significantly decreased the incidence of skin spot, black scurf, silver scurf and gangrene in tubers stored at Sutton Bridge in 1977–78. After lifting the experiment in 1978 samples from treated and untreated seed were treated with thiabendazole (40 g a.i./t) before storage (ware treatment). Skin spot was decreased more by seed treatment than ware treatment whereas silver scurf was controlled best by ware treatment. Incidence of black dot was not consistently affected by seed or ware treatment.     

The effect of disinfecting and bruising seed potatoes on the incidence of dry rot (Fusarium caeruleum (Lib.) Sacc.)

The effect of bruising and of disinfecting ('dipping') seed potatoes with a proprietary organo-mercury preparation on the incidence of dry rot in them was tested in field trials during three seasons. The tubers used were of the susceptible variety Ninetyfold, taken from crops grown in contaminated soil, harvested immature in July to early August each year under farm conditions, and stored in boxes.
Seed tubers not deliberately bruised, whether dipped or not at lifting time, remained practically sound until planting time in the fallowing season, if left undisturbed in their boxes.
Tubers deliberately bruised, either at digging time or 1-2 weeks later, but not dipped, developed severe dry rot with few exceptions. The disease had run its course by mid-October. When undipped, sound tubers were bruised in October, they contracted severe dry rot, but dipping such tubers immediately before bruising reduced the loss satisfactorily in five out of six trials.
Tubers bruised at digging time and immediately dipped suffered little from dry rot in almost all cases. Delayed dipping of bruised tubers checked the disease in some trials but not in others. Seed tubers severely bruised 1-2 weeks after being dipped remained practically sound except in one instance, whereas tubers severely bruised approximately 3 months after being dipped, subsequently developed severe dry rot in four out of six tests, unless they had been redipped immediately before they were bruised.
Inoculation of healthy tubers with soil samples showed that the fungus is widely distributed in potato fields in Cheshire. Dipping killed all, or almost all, of the fungus in the soil adhering to the seed tubers.
The results are discussed and suggestions are made for further investigations and for practical control measures.     

A Quantitative Polymerase Chain Reaction Assay for the Detection of Polyscytalum pustulans, the Cause of Skin Spot Disease of Potato

Skin spot disease of potato caused by the pathogen Polyscytalum pustulans is likely to become more important with the withdrawal of 2-aminobutane as a fungicide, and new methods of control will need to be found. As part of a disease control strategy, it will be necessary to study the disease in more detail, to utilize host resistance and to identify stocks where problems are likely to arise. Existing methods for the detection and quantification of P. pustulans are time-consuming and require specific expertise. Real-time PCR assays have been developed for many pathogens of potato and have subsequently been used as tools for the study of the epidemiology and control of disease. The development of a real-time PCR assay for the detection and quantification of P. pustulans is described. The specificity of the assay was demonstrated and detection was shown to be reliable at levels as low as 20–250 fg/ μ l DNA, (equivalent to 60–680 pg DNA/g) in soil and on symptomless tubers at attogram (ag) levels. These values are in line with previously developed tests.     

Fungus diseases on potato seed tubers planted in England and Wales, 1963-76

During 1963-76 samples of potato tubers from commercial seed stocks of cvs King Edward (14 yr), Pentland Crown (9 yr), Majestic (7 yr), Pentland Dell (3 yr), Record and Arran Pilot (2 yr) were received from farms in England and Wales. Fifty tubers from each sample were examined macroscopically for fungus diseases and eyes were excised from a 20-tuber sub-sample, incubated and examined for pathogenic fungi; 50 tubers were stored on trays to sprout and examined for diseases and sprouting in May and in most years samples of 50 tubers were wounded by dropping onto expanded metal, stored at 5° C and examined for gangrene and dry rot after 12 wk. Amounts of disease varied between years and during 14 yr black scurf and powdery scab on King Edward tended to increase and skin spot and late blight decrease. On average 44% of King Edward tubers were affected with skin spot, 25% with black scurf and 16% with powdery scab. Gangrene affected 5% of tubers and 97% of the isolates from rots were identified as Phoma exigua var. foveata. Wounding tubers increased the incidence of gangrene three-fold. During 1963-69 late blight affected 2% of King Edward tubers but fewer in later years and in other cultivars. Majestic had most common scab (44% tubers) and Arran Pilot most dry rot (9% tubers) and this disease was increased by wounding tubers. Conidiophores of Helminthosporium solani (silver scurf) were more common on excised eyes of Pentland Crown, Record and Arran Pilot than of other cultivars, and isolations from verticillate conidiophores that developed on the side of incubated eye plugs of King Edward and Majestic stocks gave pure cultures of Verticillium tricorpus (78%), V. nigrescens (9%) and V. nubilum (3%). Proportions of tubers with different diseases were affected by their country of origin; Scottish seed had most skin spot and gangrene, Irish seed most powdery scab and English seed most common scab, late blight and H. solani. There was also evidence of differing disease incidence in seed from different geographical areas in Scotland and England. Up to half the King Edward and Pentland Crown stocks examined in 1975 and 1976 were derived from stem cuttings and average amounts of diseased tubers were similar to those in stocks not derived from stem cuttings. Annual and cultivar differences in disease incidence and effects of date of receipt of seed on farms are discussed.     

Transmission of Fusarium solani var. coeruleum and F. sulphureum from seed potatoes to progeny tubers in the field

In experiments in three years, seed tubers were inoculated before planting with either Fusarium solani var. coeruleum or F. sulphureum to initiate a rot, or were contaminated by dipping in soil slurries containing spore suspensions of one or other of the pathogens. Transmission to progeny tubers was tested by uniformly wounding and incubating tubers and by dilution plating of soil samples. In two years, transmission of F. sulphureum was greater from highly contaminated than from rotting seed and was greater on cv. Pentland Crown than on cvs Desiree and Maris Piper. F. solani var. coeruleum appeared to be transmitted most readily from rotting seed and Maris Piper was the cultivar most extensively contaminated. In experiments with different harvest dates, transmission of both fungi from highly contaminated seed could be detected by late June or mid-July. More progeny tuber wounds rotted in F. sulphureum than in F. solani var. coeruleum plots and in one year, F. sulphureum caused more rots on cv. Record than on cv. Maris Piper. These differences between the pathogens may be related to their differing abilities to sporulate underground on the surface of seed tubers and on stem bases.     

Pathogenesis of Potato Gangrene Caused by Phoma exigua var. foveata. I. Location of Phosphatase Activity in Diseased Tuber Tissue

The nitrocellulose blotting method was adapted for locating phosphatase activities in gangrene‐diseased tuber tissue. Adenine and glucose phosphates were used as enzyme substrates. A strong dephosphorylation of high‐energy phosphates took place in the UV‐fluorescent tissue adjacent to dry rots caused by Phoma exigua var. foveata, but the enzyme remained at a low activity in the other areas of the diseased tuber. Phosphatases were probably induced by P. exigua var. foveata, and intense dephosphorylation of high‐energy compounds leads to the death of potato tuber tissue.     

The effect of treating seed potato tubers with benzimidazole, imidazole and phenylpyrrole fungicides on the control of rot and skin blemish diseases

Over 6 yr the effectiveness of imazalil, prochloraz and fenpiclonil, applied either alone or in a mixture, in controlling gangrene, dry rot, skin spot and silver scurf on potato tubers in store was compared with that of 2-aminobutane and thiabendazole. An assessment was also made of their efficiency in controlling the development of these diseases on the daughter tubers of seed tubers treated at harvest or before planting. Overall, 2-aminobutane was more effective in controlling gangrene (Phoma foveata) in store than the spray-applied fungicides. Deposits of imazalil, thiabendazole and fenpiclonil were greater when sprays were applied with an electrostatic sprayer than with a hydraulic sprayer. The opposite was found with the mixture of prochloraz Mn and tolclofos-methyl. More effective gangrene control was associated with the highest deposits. Fenpiclonil and the mixture of thiabendazole and imazalil were more effective in controlling dry rot (Fusarium solani var. coeruleum) than imazalil alone. The development of dry rot was, however, increased by 2-aminobutane treatment on eight out of 14 stocks. 2-aminobutane gave the greatest reduction (83%) in the severity of skin spot during storage whereas thiabendazole alone, and the mixture of thiabendazole and imazalil, gave mean reductions of 70% and 65% respectively. This mixture and fenpiclonil gave the greatest reduction in the severity of silver scurf although, in general, reductions in silver scurf with fungicide treatment were less than with skin spot. The type of sprayer used to apply a fungicide did not affect the effectiveness of the fungicides in controlling either skin spot or silver scurf on tubers in store, or on the daughter tubers. The incidence of gangrene and dry rot on daughter tubers was not reduced consistently by fungicide treatment of seed tubers of the six stocks tested. However, the severity of skin spot and silver scurf was reduced by fungicide treatments of all eight stocks but the reduction in disease was greater for skin spot than for silver scurf. All fungicides gave reductions in the severity of skin spot, and fenpiclonil and the mixture of thiabendazole and imazalil were the most effective for silver scurf. The effectiveness of the fungicides in controlling these diseases was similar for seed treated at harvest and that treated before planting.     

EXPERIMENTS ON THE EFFECT OF CHLORINATED NITROBENZENES ON THE SPROUTING OF POTATO TUBERS

The addition of a small quantity of a dust containing pentachloronitrobenzene to clamps of certain early and second-early potato varieties has a marked effect in reducing the amount of sprouting which takes place over winter and in protecting the sprouts from damage by the fungus Rhizoctonia Solani.
A series of yield trials carried out over six seasons has shown that tubers which have been stored in clamps in presence of the dust give yields equal to those of chitted tubers.
A dust containing a tetrachloronitrobenzene applied to potatoes kept in a closed space represses sprouting to a very pronounced degree. The possible use of this dust for conserving ware potatoes is suggested.     

Evaluation of selective media for the isolation of Fusarium solani var. coeruleum and Fusarium sulphureum from soil and potato tuber tissue

A new selective medium containing pentachloronitrobenzene and 2-aminobutane (the PAB medium) was developed for soil-dilution plate enumeration of fungal propagules of Fusarium solani var. coeruleum and F. sulphureum from field soil. Growth of ‘weed-fungi’ was less on the PAB medium than on the previously developed F. solani var. coeruleum isolation medium (the PM70 medium) and significantly more propagules of F. solani var. coeruleum were detected. Propagule counts (x) of F. solani var. coeruleum and F. sulphureum from the PAB medium, after log₁₀ (x/10 + 1) transformation, were linearly related to the angular transformation of measurements of soil infectivity from the tuber bait method using the susceptible cv. Catriona. Slopes from disease-inoculum regressions for F. solani var. coeruleum and F. sulphureum in November and May were similar and this suggests that the mode of pathogenic action of F. solani var. coeruleum and F. sulphureum was similar. Recently harvested tubers, inoculated in November, however, were more resistant to infection by both pathogens than stored tubers inoculated in May. Storage of air dry soil at 4°C for 6 months reduced the population of F. sulphureum but not the population of F. solani var. coeruleum. Whereas the PAB medium is recommended primarily for use in the isolation of F. solani var. coeruleum and F. sulphureum from field soil, the PM70 medium appears to be more suitable for recovery of these pathogens and others, including Phoma exigua var. foveata from diseased tuber tissue.     

The Influence of Glycoalkaloids and Impact Damage on Resistance to Fusarium solani var. coeruleum and Phoma exigua var. foveata in Potato Tubers

Potato glycoalkaloids were determined in tubers of a wide range of genotypes in order to study their influence on resistance to storage rots caused by Fusarium solani var. coeruleum and Phoma exigua var. foveata. No relationships were found. This indicates that breeders selection for low glycoalkaloid potato clones is not in conflict with the demands for high levels of resistance to these pathogens. It was also observed that genotypes susceptible to skin-breaking types of, damage were more easily infected than genotypes susceptible to other types of damage.     

The influence of potato harvesting and grading machinery on contact spread of Phoma exigua var. foveata on tubers

The incidence of wounds infected by Phoma exigua var. foveata was increased if freshly damaged tubers (recipients) were shaken in a bag with diseased tubers (donors) to simulate the tuber-to-tuber contact that occurs during potato handling. An increase in the number of gangrene rots on damage points also occurred if the recipient tubers were wounded after contact with the diseased tubers, rather than before, and when the donor tubers were heavily infested with P. exigua var. foveata but were free of gangrene lesions. Increasing the proportion of donor to recipient tubers increased the percentage of infected wounds on recipients. Increased incidences of infection in recipient tubers also occurred after they had been passed over an elevator digger when it was lifting stocks of tubers heavily infested with P. exigua var. foveata. When spores of an E +ve isolate of P. exigua var. foveata were sprayed onto the webs of manned potato harvesters, tubers harvested immediately after developed gangrene rots from many of which the E +ve isolate was cultured. An E +ve isolate was also used to demonstrate the transfer of P. exigua var. foveata inoculum from tubers onto soil on riddles of a potato grader and from these soil-coated surfaces onto other tubers during grading.     

Effects of delaying fungicide treatment of wounded potatoes on the incidence of Fusarium dry rot in store

Potato tubers artificially inoculated with Fusarium solani var. coeruleum or F. sulphureum 3 months after harvest were uniformly wounded and held at 5, 10 or 15°C for up to 21 days before immersion in fungicide suspensions. Holding tubers for 14 days at 15°C (curing conditions) or at 5°C did not alter the incidence of dry rot subsequently developing on tubers stored at 10°C, and holding tubers for up to 21 days at 15°C slightly increased disease caused by both pathogens. Thiabendazole, imazalil and prochloraz applied to tubers immediately after wounding almost completely prevented dry rot. Treatment after 3 days was less effective and the amount of disease increased with further delay; fungicides were more effective on tubers held at 5°C than at 10 or 15°C before treatment and storage, and efficacy of the fungicide was decreased by increasing the amount of inoculum on tubers. Wounds became less susceptible to infection by F. solani var. coeruleum and F. sulphureum when tubers were held at 15°C before inoculation, and the incidence of rots was decreased by 70–80% by delaying inoculation for 7 days. Treating tubers with dichlorophen immediately after wounding slightly increased the disease. The effects of fungicide treatment, curing conditions and wound healing are discussed.     

Effects of treating seed potatoes from commercial and stem cutting stocks with benomyl, thiabendazole and 2-aminobutane on yield and disease

Potato seed tubers of six cultivars from commercial stocks and from stocks derived from stem cuttings (healthier seed) were fumigated with 2-aminobutane 2 wk after lifting or treated with benomyl or thiabendazole in January. 2-aminobutane prevented skin spot and gangrene developing on treated tubers. Experiments were planted at Rothamsted (clay with flints soil) and at Woburn (sandy loam soil) in 1973–75. Healthier seed produced more stems/plant than commercial stocks and yielded on average 8% more at Rothamsted in 1973 and 1974 and respectively 5 and 10% more at Woburn in 1973 and 1975. Seed treatments did not consistently affect stem numbers or increase yield although all treatments tended to decrease tuber size. Infection of stem bases and tubers by Polyscytalum pustulans and Rhizoctonia solani was usually less from healthier than from commercial seed and was decreased by benomyl and thiabendazole in 1973 and 1974. Infection by Helminthosporium solani of the skin around tuber eyes was greater from healthier than from commercial seed but was decreased by benomyl and thiabendazole. 2-aminobutane sometimes decreased infection of tubers by P. pustulans and R. solani but neither of stem bases nor of tubers by H. solani. Gangrene on tubers uniformly wounded at lifting was not consistently affected by seed source or seed treatment. Treating seed with benomyl or thiabendazole in 1975 decreased skin spot and black scurf in tubers stored until March 1976. These treatments also decreased silver scurf on the produce of commercial seed at Rothamsted but gangrene was not consistently affected by seed treatments.     

Effects of skin spot (Oospora pustulans) on potatoes

King Edward and Majestic seed potatoes selected as ‘clean’ (macroscopically symptomless), moderate and severe according to the extent of skin spot were planted in field experiments at Rothamsted between 1964 and 1968. Usually crops from ‘clean’ and moderately infected seed did not differ detectably in growth or yield. Plants from severely infected seed tubers emerged more slowly, had fewer stems and yielded less (King Edward 20 %, Majestic 13 %). Seed infection also affected tuber size distribution; severely infected seed of King Edward yielded almost 4 tons/acre less of 1 1/4-2 1/4 in tubers and Majestic, 1 ton/acre less of these and 2 tons/acre less 2 1/4-3 1/4 in tubers. However, the total yield from diseased seed stocks was only slightly less (King Edward, o-6 ton/acre and Majestic o-8 ton/acre) than the yields from the ‘clean’ tubers selected from them. Seed severely infected by Oospora pustulans often increased infection of the progeny tubers, and usually decreased their infection by Rhizoctonia solani and sometimes by Helmintho-sporium solani. Another series of experiments compared King Edward seed tubers classified according to the number of live eyes showing in March. Seed with one, two, three and more live eyes yielded equally. About half the tubers without live eyes in March eventually produced plants, but late, with few stems and giving only half the yield of seed with three or more live eyes. Surprisingly, the progeny tubers from seed without live eyes were least infected by O. pustulans, R. solani and H. solani. Progenies of King Edward and Majestic seed from a common source grown on seven widely separated farms were infected more in 1963 than in 1964, but in each year infection differed widely between farms. Often where O. pustulans was common, R. solani was scarce and vice versa. By contrast, when King Edward stocks very differently infected by O. pustulans were grown at Rothamsted their progenies were almost uniformly infected by O. pustulans and R. solani.     

DRY-ROT DISEASE OF THE POTATO

A study has been made of the incidence of dry rot in seed potatoes in relation to commercial handling methods, and it has been shown that infection following grading by mechanical reciprocating riddles fitted with bare wire screens is more serious than that following normal handling at harvest or during transport. Hand picking, hand riddling and the use of rubber-spool graders or rubber-coated screens reduced infection by varying amounts. The higher incidence of the disease in 'transported' as against 'home-saved' seed is regarded as being due to machine grading of such stocks some months prior to planting.
Contamination of stores or boxes has not been found to play any material part in the spread of infection under commercial conditions unless tubers are roughly handled during storage. As contact infection has been found to be rare, it is considered that the practice of 'picking over' stocks during the winter to remove diseased tubers is unnecessary and may lead to further infection.
In addition to wounds, lesions of both blight and powdery scab, but not common scab, have been found to be a means of infection.     

Black scurf and stem canker of potato (Corticium Solani Bourd. & Galz.): Further field studies on the use of clean and contaminated seed potatoes and on the contamination of crop tubers

Black scurf and stem canker of the potato were investigated in field trials in contaminated soil at Warburton, Cheshire, to determine the effect of planting clean and contaminated seed (vars. Arran Banner and Majestic) on crop yield and on the contamination of the crop tubers.
Black-scurf contamination was prevalent on crops grown from clean seed and was not significantly different from that occurring on plots planted with contaminated seed. It was severe on early-dug tubers, but was more severe on late-dug tubers. Produce grown with and without stable manure was heavily contaminated.
Contaminated seed caused a check to tuber formation and an appreciable increase in stem canker and in the number of primary shoots killed. Nevertheless, the yields from the manured, contaminated seed plots were satisfactory and were not lower than those from the clean-seed plots. There was no relation between the yield and the amount of black scurf on the produce.
The results of a trial made in a field which had been in grass for at least 43 years, suggested that the soil contained little, if any, Corticium Solani. Heavy contamination occurred on tubers grown under relatively dry soil conditions.
In all the trials misses and wilted shoots caused by Corticium were rare and there was no premature yellowing or death of the haulms on the clean or contaminated seed plots.
The evidence obtained in the four seasons from 1941 to 1944 indicates that in this country satisfactory yields of early maincrop and maincrop varieties may be obtained despite the prevalence of C. Solani in the soil and on the seed, provided the soil and cultural conditions are reasonably good.