Effects of dual infections of Puccinia hordei and Erysiphe graminis on barley, cv. Zephyr

Seed tubers of the varieties King Edward, Majestic and Pentland Crown selected as ‘clean’ (lesion-free), moderately, or severely affected by gangrene lesions were planted in field experiments. Infection delayed plant emergence, increased the number of stems/plant, sometimes caused gaps in crops and was associated with increased blackleg. On average severely affected seed yielded 20% less than ‘clean’ seed. Seed infection also increased the proportion of tubers in smaller size grades so that crops from severely infected King Edward seed averaged 1·4 ton/acre (3·5 t/ha) less small ware and 2·5 ton/acre (6·3 t/ha) less large ware than ‘clean’ seed. With Majestic, small ware was increased (0·7 ton/acre (1·8 t/ha)) and large ware decreased (4·4 ton/acre(11·0 t/ha)); Pentland Crown was similarly affected (small ware increased 0·8 ton/acre (2·0 t/ha); large ware decreased 3·9 ton/acre (9·8 t/ha)). In eight of twelve experiments unselected diseased stocks yielded significantly less than ‘clean’ tubers. Other experiments compared seed stocks with different proportions of gangrene-infected seed tubers. Yields decreased as the proportion of diseased seed tubers increased, but differences were significant only when more than 60% were affected. Surprisingly, yields from ‘clean’ tubers also decreased as the proportion of diseased tubers increased in the stocks from which they were selected. Gangrene on progeny tubers after storage was not always related to the amount of gangrene visible on the seed. It was increased by riddling or wounding and decreased by dipping tubers in organo-mercury fungicide before or soon after wounding.     

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.     

Factors affecting the field infection of potato tubers of different cultivars by blight (Phytophthora infestans)

The relationships between rain and blight (Phytophthora infestans) were studied in unsprayed crops of cultivars differing widely in foliage and tuber susceptibility. The occasions when tubers were infected depended on rain and not cultivar, but numbers of tubers infected after rain was affected by the blight susceptibility of the cultivar. Infected tubers were first found when less than 5 % (BMS key) of the potato foliage was infected but few fresh infections occurred when 50–75% of the foliage had been destroyed. Some tubers were infected after 8 mm rain (tubers near the surface with even less) but large increases in numbers of tubers infected usually occurred only after 25 mm or more had increased soil moisture to above ‘field capacity’ around the tuber for at least 24 h. The most susceptible cultivars Ulster Ensign and Arran Banner had all plants with some tuber blight, and some plants with all tubers affected and often many lesions per tuber. Cultivars of intermediate susceptibility, King Edward and Up-to-Date, had some plants without blighted tubers, many with a few and very few with all. The more resistant cultivars Majestic and Arran Viking had many plants without infected tubers and many lesions that aborted while still necrotic threads, so that the fungus did not spread. Most infections occurred through tuber eyes, lenticels or sometimes growth cracks. The distribution of blight lesions on tubers differed in the different seasons, for example, lenticels were most commonly infected on Arran Banner and Ulster Ensign and eyes on King Edward, Majestic and Arran Viking. In late or slowly developing attacks, lesions on stems became more numerous and larger than in fast, early attacks and were prolific sources of spores on King Edward and Up-to-Date but not on Majestic and Arran Viking. Because much rain water runs down the stems of Up-to-Date and King Edward, stem lesions can provide an important source of inoculum for tubers.     

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.     

The role of water in the spread of Phytophthora infestans in the potato crop

During falls of rain before 5% of the haulm was destroyed by blight, most rain water was deposited on soil at the bottom of the ridge, and least at the top in crops of King Edward, Majestic, and Up-to-Date potatoes. Afterwards less water was trapped half way up the side of the ridge than above or below. Water was also channelled down the stems and, while these were upright, was deposited in the channel often formed around the stem bases by wind movement. Fewer than 500 sporangia per ml. were seen in water collected when blight was present in the crop. Growing tubers of Ulster Ensign and King Edward, inoculated with P. infestans, infected healthy tubers less than 1–3 cm. away, when the soil water content was greater than 20%, and sporulation on the inoculated tubers was maximal. Similarly inoculated Up-to-Date and Majestic tubers sporulated less abundantly and failed to infect healthy tubers. Sporulation also occurred on inoculated seed tubers, although infection of the haulm from these tubers could not be confirmed. Water movement from the seed tuber region to the furrows may occur under suitable conditions.     

Effects of black scurf (Rhizoctonia solani) on potatoes

King Edward and Majestic seed tubers, selected as ‘clean’ (macroscopically symptomless), moderate and severe according to the extent of black scurf, were planted in field experiments at Rothamsted between 1964 and 1968. Seed infection sometimes delayed plants emerging but did not affect final plant populations. Crops from severely diseased seed yielded, on average, 7% less than ‘clean’ tubers (King Edward 6–8% less and Majestic 0–20% less). Seed infection affected tuber size distribution; compared with ‘clean’ seed, severely infected King Edward seed yielded slightly more chats (< 1 ½ in, 3.8 cm) and 1.5 ton/acre (3.8 t/ha) less large tubers (2 ¼–3 ¼ in, 5.7–8.3 cm). The effects were similar with Majestic although differences were smaller. However, total yields from diseased stocks (unselected) seldom differed significantly from the ‘clean’ tubers selected from them. Crops from moderately and severely diseased seed had more Corticium on stems and black scurf on tubers and usually less Oospora pustulans than from ‘clean’ seed.     

The induction of top-roll symptoms on potato plants by the aphid Macrosiphum euphorbiae

In the field, caged potato plants of King Edward and Majestic cultivars infested with the potato aphid Macrosiphum euphorbiae developed top-roll symptoms, the proportion of affected plants increasing with the size and persistence of the aphid population. Yield of tubers from plots in which 90% of the plants had top-roll symptoms was 40% less than that from control plots; yield of saleable ware was even less. Foliage produced after the aphids had been killed was symptomless even when it arose from the axil of an affected leaf. Caged field plants treated with phorate granules to prevent aphid attack did not develop top-roll. Prolonged infestation of Pentland Crown, Majestic and King Edward plants by M. euphorbiae in a glasshouse induced rolling of upper leaves similar to top-roll of field plants. Experimental results suggest that rolling was directly attributable to heavy attack by M. euphorbiae, not to an aphid-transmitted pathogen.     

Laboratory studies of potato tuber resistance to infection by Phytophthora infestans

Eyes and lenticels on tubers of cvs King Edward, Record, Maris Peer and Majestic gradually became more resistant to infection by Phytophthora infestans as the growing season progressed. Lenticels of Record and Majestic, and eyes of Maris Peer and Majestic were more resistant than those of King Edward. Differences in resistance between cultivars were apparent at all concentrations of inoculum.     

The phenology of skin spot (Oospora pustulans Owen &Wakef.) and other fungal diseases of potato tubers

Underground stems and roots of the potato varieties King Edward and Majestic became more severely infected by Oospora pustulans (skin spot) as the growing season advanced. Tubers became infected at their initiation in June and July but the proportion of infected eyes usually increased during both the growth of the crop and bulk storage. Some buds on tubers in bulk stores died in December, but few were killed until after January, when attacks increased until by March about a quarter of Majestic eyes were usually dead. Boxing (chitting) not only prevented most eyes from dying but also prevented much skin spotting on tubers. At High Mowthorpe, Majestic seed unboxed or boxed in March yielded, respectively, 3 and 1 ton/acre less than seed boxed by January. Early boxing increased the yield of seed–sized tubers but did not affect the infection of progeny tubers. Dipping Majestic seed tubers in organo–mercurial fungicide and then boxing soon after lifting decreased their infection by O. pustulans; it also lessened infection on plants they produced and increased the yield of progeny seed–sized tubers. However, dipping, even in successive years, had little effect on the infection of progeny tubers by O. pustulans. Rhizoctonia solani and Helminihosporum atrovirens became increasingly prevalent on tubers during growth after July and during bulk storage. H. atrovirens was decreased by early boxing or dipping tubers in fungicide. Verticillate conidiophores often grew on incubated tuber plugs but their prevalence was affected by past storage. The results suggest that some of the benefit for chitting may be from disease control, it was not possible to measure the extent to which benefits from chitting depend on effects on disease or on the physiology of the tuber.     

Healthier seed potatoes. I. Effects of inoculating stem cutting stocks with Polyscytalum pustulans and Rhizoctonia solani on growth, yield and disease

Potato seed tubers of seven cultivars derived from stem cuttings in 1965 (healthier seed) were grown in 1969–72 at two sites, one clay with flints soil (Rothamsted) and the other sandy loam soil (Woburn). Inoculating sprouted tubers at planting with Polyscytalum pustulans did not affect the number of stems/plant or total yield but increased stem base and tuber infection. The yields of large tubers (57–83 cm) were increased and small tubers (>57 cm) decreased, indicating a decrease in tuber numbers. Rhizoctonia solani inoculated at planting decreased numbers of stems/plant and yield by up to 14% at Woburn but not at Rothamsted. At both sites, yields of large tubers were increased with cvs Majestic and Record and decreased with Pentland Crown. Stem canker and tuber infection were increased but infection was also prevalent on tubers from non-inoculated seed at Woburn. When both pathogens were inoculated together yields of large tubers were increased in cvs King Edward, Majestic and Record and decreased in Pentland Crown. Infection of stem bases and tubers was sometimes less than when either pathogen was inoculated singly.     

The control of common scab of potatoes by irrigation

Irrigation applied early during tuberization significantly decreased scab on tubers of Majestic, King Edward and Record, but not on Pentland Crown, a resistant cultivar, where tubers had little scab even from the driest soil. There was little benefit from keeping soils wetter than 0.6 in (15.24 mm) soil moisture deficit (S.M.D.) for more than 3 weeks after tuber initiation, and irrigation to 0.8 in (20.32 mm) S.M.D. gave economic control. Irrigating only after the ‘marble stage’ increased yield but the tubers were scabbed. Irrigation did not alter the rate eyes (nodes) separated from the apical bud but speeded the swelling of tubers.     

Black scurf and stem canker of potato (Corticium Solani Bourd. & Galz.)

Black scurf and stem canker of the potato was investigated in field trials on infected soil at Warburton, Cheshire, to study the effect of planting clean and contaminated seed, and the relation between the dates of planting and lifting and the contamination of the crop tubers.
Black scurf was prevalent on crops grown from clean seed but was more severe on crops raised from contaminated seed. Under conditions favourable for the disease the yields from clean and contaminated seed were satisfactory and were not significantly different. Young shoots of plants from contaminated seed were severely attacked and tuber formation was checked; these results were not obtained on clean seed plots.
More black scurf occurred on late-dug crops; on early-dug plots the disease, although almost absent in 1941, was prevalent in 1942. Late planting failed to reduce appreciably the amount of black scurf.
Variety trials included Arran Banner, Kerr's Pink (two vigorous varieties), King Edward and Majestic (two less vigorous varieties). All were heavily infected and each recovered well from an attack on the young shoots.
Inoculation of the seed at planting time did not affect the results; in all trials, misses and wilted shoots caused by Corticium were rare and there was no relation between the yield and the amount of black scurf on the crop tubers.
The results indicate that the disease causes little, if any, loss under farm conditions.     

The effect of Erwinia carotovora subsp. atroseptica (blackleg) on potato plants. I. Growth and yield of different cultivars

The effect of planting seed tubers inoculated either near the stolon attachment (heel end), among the eyes at the apex (rose end) or mid-way along the tuber with Erwinia carotovora subsp. atroseptica, was assessed in terms of growth of the plant, disease symptoms and yield. Invariably rose-end inoculation had the greatest and heel-end the least effect in decreasing yield when compared with uninoculated plants. Cultivars Majestic and King Edward were the most susceptible, Pentland Crown showed some resistance to invasion of stems (blackleg) although plant vigour, expressed in terms of plant height and stem number was affected and Maris Piper was the most resistant.     

ROGUING POTATO CROPS FOR VIRUS DISEASES

Removing virus-infected plants from plots of Majestic potatoes at Rothamsted on 2 July 1947 did not reduce the spread of leaf roll but reduced rugose mosaic (potato virus Y) to about one-fifth of that in plots rogued on 21 July or left unrogued. Roguing Arran Pilot potatoes on 16 June or 2 July reduced leaf roll to five-sixths of that in unrogued plots; roguing on 16 June reduced rugose mosaic to about half that in plots rogued on 2 July, and about a quarter of that in unrogued plots. Lifting Arran Pilot potatoes in mid-August reduced virus diseases to about two-thirds.
Roguing flattened the gradient (decrease in percentage plants diseased with increasing distance from the source of infection) with rugose mosaic, but had little effect with leaf roll. Evidently any plants prevented by roguing from contracting virus Y were near the initially infected plants.
In 1948, Majestic and King Edward potatoes at three places were rogued during 22–24 June and tubers were dug during 28–30 July and again at the end of the season. Leaf roll spread more in Majestic than in King Edward, and rugose mosaic spread more in King Edward. Roguing reduced the spread of both by about one-fifth at Rothamsted, but had no effect at Sutton Bonington. At Bretton, in the Derbyshire hills, roguing had no effect on leaf roll, but prevented the spread of rugose mosaic.
The small benefit occasionally achieved by roguing in the ware-growing districts of England does not make the practice economically worth while.     

Effects of potato leafroll virus on the crop processes leading to tuber yield in potato cultivars which differ in tolerance of infection

The effects of infection with potato leafroll virus (PLRV) on the four crop processes leading to tuber fresh weight yield were examined in field plots of four cultivars (Montana, Pentland Crown, Maris Piper and King Edward) differing in tolerance to infection. Averaged across cultivars, infection decreased yield by 50%. This decrease was equally due to less light (total solar radiation) being intercepted, a lower efficiency with which intercepted light was converted into dry weight, and a smaller proportion of dry weight being partitioned to tubers. Dry matter content of the tubers was also diminished but to a lesser extent. The main difference between the cultivars in their response to infection was in the partitioning of dry weight. In Montana and Pentland Crown, harvest index was decreased by 15% in infected plants, whereas in the less tolerant cultivars, Maris Piper and King Edward, it was decreased by 25%. The decline in photosynthetic performance of Montana, a cultivar with slightly earlier maturity than the other three, was delayed in PLRV-infected plants. Effects on number of daughter tubers essentially reflected those on yield to the extent that average tuber weight did not change in Maris Piper, was one third less in King Edward, and the change was intermediate in Montana and Pentland Crown.     

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 small doses of nematicides on migratory root-parasitic nematodes and on the growth of sugar beet and barley in sandy soils

Smaller amounts of D–D (6–12 gal/acre) (68–135 1/ha) or ethylene dibromide (9 gal/acre) (100 1/ha) than are customarily used to disinfest field soils killed many root-parasitic nematodes (Trichodorus, Pratylenchus, Tylenchorhynchus and Longidorus attenuatus) when injected 6–8 in (15–20 cm) deep during early autumn in rows 10 in (25 cm) apart in well-drained sandy soils. They also increased the yield of sugar beet grown in fields infested with Trichodorus or Longidorus attenuatus, without affecting sugar percentage or juice purity of the roots, and in some places increased the yield of barley grown after the beet. D–D was much less effective when injected 8–12 in deep during late autumn or winter. Increasing nitrogen dressings to the seedbed from 1·5 to 3 cwt/acre (188 to 376 kg/ha) increased sugar beet yield in one field, decreased it in another and decreased juice purity in both. In two other experiments extra nitrogen did not affect sugar beet yield. Even smaller amounts of the nematicides ‘placed’ in the rows, before or after sowing sugar beet in them, killed many of the nematodes and also increased sugar yield. Phytotoxic nematicides can be placed in the rows during autumn, winter or spring but placement is simpler during spring, when the treated rows are indicated by the position of the marks of the tractor wheels left when the nematicide was applied. When applied during autumn or winter, the rows need to be indicated by drilling wheat or grass.     

EFFECTS OF PREVIOUS WHEAT CROPS, SEED-RATE AND NITROGEN ON EYESPOT, TAKE-ALL, WEEDS AND YIELDS OF TWO VARIETIES OF WINTER WHEAT: FIELD EXPERIMENT 1954-56

In a replicated field experiment mean yields of wheat from plots that, in the preceding 2 years, had carried oats, beans or potatoes were 39.2 and 42.6 cwt. per acre in 1954 for Holdfast and Cappelle, respectively; 42.8 and 55.8 in 1955 and 34.9 and 49.6 in 1956. Previous wheat crops had more effect than any other treatment in increasing the incidence of eyespot, take-all and weeds and in decreasing the number of ears per unit area and the yield of grain. In 1956 on plots carrying the first, second and third successive wheat crops the percentages of straws with eyespot were respectively 12, 54 and 42 and with take-all 0.1, 1 and 16. Cappelle was less severely infected by eyespot than Holdfast. The second and third successive wheat crops yielded an average of 23.3 cwt./acre less than the first wheat crop. Cappelle consistently yielded more than Holdfast, the mean difference being 13.8 cwt./acre after potatoes but only 3.8 cwt./acre after two wheat crops. The higher seed-rate gave an average increase in grain yield of 3.3 cwt./acre; but where eyespot and take-all were both severe the lower seed-rate yielded as much total and more dressed grain than the higher. Wheat given a spring top dressing of 6 cwt./acre Nitro-Chalk yielded an average of 4 cwt./acre more grain than wheat given 3 cwt./acre.     

FIELD EXPERIMENTS ON POTATO SKIN-SPOT DISEASE CAUSED BY OOSPORA PUSTULANS OWEN & WAKEF

In experiments on skin-spot disease of potatoes, caused by Oospora pustulans Owen & Wakef., Kerr's Pink proved the most susceptible of twenty-four commercial varieties; Arran Banner, Majestic and King Edward were highly susceptible; Home Guard and Golden Wonder were highly resistant.
Thymol and tetrachlornitrobenzene, applied as dusts to pits of potatoes at lifting, did not give satisfactory control of the disease. The efficacy of an organo-mercury dip treatment at lifting time was confirmed.
The incidence of the disease was considerably decreased by storing tubers in boxes instead of in pits, by digging the crop about a month before normal harvest, or by cutting the haulms at this time and digging at the normal time.
Ware tubers showed a significantly higher infection than seed tubers of the same crop, indicating that susceptibility of tubers increases towards maturity.
The difference in eye infection between varieties may not be related to that of general superficial infection. It is suggested that, in future, the assessment of varietal susceptibility should be based upon the degree of eye infection, which is in practice the measure of the economic importance of the disease. Tubers having pustules at or near all the eyes gave markedly reduced sprout emergence and plant stand.     

A comparison of Fusarium avenaceum and Fusarium caeruleum as causes of wastage in stored potato tubers

Fusarium avenaceum is reported for the first time as a cause of rotting of potato tubers in Britain. The progress of rotting in tubers infected with F. avenaceum has been compared with dry rot due to F. caeruleum in the laboratory, clamp and potato store. Of the four varieties, Majestic, King Edward, Doon Star and Arran Pilot, tested for susceptibility, King Edward was the most susceptible to F. avenaceum and Doon Star to F. caeruleum.
Optimum temperatures for growth on potato-dextrose agar were 20-25 C. for F. avenaceum and 20 C. for F. caeruleum ; maximum temperatures were > 30 and 30 C. respectively. For infection of wounded potato tubers, cardinal temperatures for F. avenaceum were similar to those on agar, but for F. caeruleum the optimum was 15 C. and the maximum 25 C. The optimum temperature for rotting tended, with both species, to be higher in the more susceptible potato varieties. At low temperatures F. caeruleum caused quicker rotting than did F. avenaceum , even though its rate of growth on agar was scarcely more than half that of the latter.
High humidity favoured rotting especially by F. avenaceum; F. caeruleum was more tolerant of relatively low humidity. Both species caused quicker rotting in the clamp than in store, even though there was no appreciable difference in mean temperature between the clamp and the store. This was attributed to the higher atmospheric humidity in the clamp.