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.     

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.     

Varietal susceptibility of potatoes to wireworm herbivory

1 Wireworms, the soil dwelling larvae of click beetles, Agriotes spp., have recently become a more prevalent pest of potatoes. The present study investigated whether potato varieties showed variable susceptibility to wireworm herbivory, and also tested whether increased susceptibility was associated with lower concentrations of glycoalkaloids. Twelve varieties were originally screened across a range of experimental scales, including laboratory and tunnel experiments and a large‐scale field trial involving over 2000 tubers. 2 In laboratory no‐choice tests, Maris Peer, Marfona and Rooster varieties were significantly more susceptible to wireworm attack, with 63% of tubers showing damage, compared with just 15% of the less susceptible varieties of King Edward, Nadine and Maris Piper. There was also greater tissue consumption and weight gain when wireworms were reared on the most susceptible varieties. 3 In choice tests, wireworms showed a significant preference for those varieties previously identified as being the most susceptible to wireworm herbivory (4.2 holes per tuber) compared with the least susceptible (1.2 holes per tuber). Similar patterns of susceptibility were seen in the field trial, although there was generally more variation in susceptibility. 4 In a tunnel experiment, Marfona and Maris Peer were significantly more susceptibile to wireworm attack (47% of tubers showing damage) compared with Nadine, King Edward and Maris Piper (27% of tubers showing damage). Although Nadine, in particular, had the highest glycoalkaloid concentrations (309.33 mg/kg) and lowest amounts of wireworm herbivory, the relationship between susceptibility and glycoalkaloid concentrations was weak, suggesting that this is unlikely to be the sole mechanism underpinning varietal susceptibility.     

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.     

Control of silver scurf (Helminthosporium solani) disease of potato with benomyl and thiabendazole

Benomyl and thiabendazole, applied as dusts to seed potatoes before chitting (sprouting), reduced the incidence of silver scurf disease on the progeny at lifting and during subsequent storage. Treatment of seed tubers 4 months prior to planting almost completely suppressed sporulation during storage, even under conditions very favourable for the growth of the fungus. The fungicides appeared not to act systemically because developing tubers grown from treated seed were just as susceptible to infection after inoculation as those from untreated. Control was due to effective inhibition of sporulation on the surface of diseased seed tubers after planting. There was no reduction in the amount of disease in subsequent generations of potato crops without further fungicidal treatment. Seed virtually free from silver scurf, produced from seed treated with fungicide in the previous year, yielded progeny with as much disease as progeny from untreated seed stocks.     

SOME FACTORS AFFECTING THE OCCURRENCE AND DEVELOPMENT IN POTATOES OF GANGRENE CAUSED BY PHOMA SOLANICOLA PRILL. & DELACR.

It is shown that gangrene is a soil- and haulm-borne disease and that infection of the tubers can take place through cuts, bruises, 'eyes' and lenticels. Low temperature and dry, dark storage were found to be most favourable to development of the disease, but rotting was more severe if the tubers had been kept at a higher temperature before infection took place. Storage at 21° C. for 10 days was sufficient to arrest rotting. Gangrene developed on all the potato varieties tested, but different varieties showed differences in susceptibility to the disease. No difference was observed in the susceptibility of early and late varieties but susceptibility increased with advancing maturity of the tubers.     

SOME OBSERVATIONS ON DAMAGE TO POTATOES BY SLUGS

The garden slug, Arion hortensis Fér., and the keeled slugs, Milax spp., are the slugs mainly responsible for primary damage to potato tubers in the south-west of England. A variety trial clearly demonstrated the existence of significant differences in susceptibility to slug attack between the three varieties tested. Arran Banner was the most damaged variety, Arran Peak the least, while Majestic occupied an intermediate position. Varietal differences in the amount and time of damage to tubers were associated with differences in the dates of maturity, and little damage occurred prior to the maturing of the tuber. The results confirm the need for prompt lifting of the potato crop on maturity, in order to avoid damage by slugs.     

THE USE OF TETRACHLORONITROBENZENE ISOMERS ON POTATOES

In laboratory and field experiments from 1952 to 1955, 2:3:5:6-tetrachloro-I-nitrobenzene (tecnazene) delayed the development of sprouts on treated potato tubers, and 2:3:4:6-TCNB retarded sprout development in two laboratory experiments. At the usual commercial rates of application the 2:3:4:5 isomer had no effect on sprout development and very little effect at higher rates. The crop from 2:3:5:6-TCNB-treated seed tubers usually contained a higher proportion of small tubers than that from untreated or 2:3:4:5-TCNB-treated seed. Marketable yield from 2:3:5:6-TCNB-treated seed was sometimes below that from untreated seed, but when this happened the yield from 2:3:4:5-TCNB-treated seed was higher.
Both isomers produced good control of dry rot, Fusarium caeruleum (Lib.) Sacc., on five varieties of potatoes, but did not produce a commercial control on Doon Star. To achieve control, the isomers had to be present at the time of infection. There was some evidence that the development of disease symptoms was delayed in treated, infected tubers. Since 2:3:4:5-tetrachloro-1-nitrobenzene has no sprout depressant action it merits further trial as a fungicidal treatment for seed potatoes.     

Variety‐specific Epidemiology of Cercospora beticola Sacc. and Consequences for Threshold‐based Timing of Fungicide Application in Sugar Beet

In Central Europe, fungicides to control leaf spot disease in sugar beet caused by Cercospora beticola are applied based on thresholds of disease incidence (DI, per cent of infected plants). As variety‐specific fungicide application was not analyzed to date, the epidemiology of C. beticola and its effect on white sugar yield (WSY) in varieties with different susceptibility were investigated at seven sites in Germany and Austria in 2004 and 2005. All varieties reached the summary thresholds 5 / 15 / 45% DI in all environments. Fitting a logistic growth curve to DI revealed significant differences among varieties. At high disease pressure, susceptible varieties reached a considerably higher disease severity (DS, per cent of infected leaf area) at harvest and a larger area under disease progress curve (AUDPC) than resistant varieties. Fitting a logistic growth curve to DS showed an increasing differentiation among varieties with time. The growth rate estimated based on the logistic growth curve was the only variable that performed equally well in differentiating varieties under low and high disease pressure. With increasing disease pressure, varieties differed considerably in WSY, but differences between susceptible and resistant varieties were significant only in some environments. The disease‐loss relation between AUDPC and relative WSY was variety‐specific. Resistant varieties had an approximately identical WSY with and without infection and compensated for negative infection effects even at higher AUDPC. Therefore, at high disease pressure, resistant varieties had a higher relative yield compared to susceptible ones. However, our results indicate that there is no need to develop variety‐specific thresholds, but resistant varieties reach the established thresholds later than susceptible ones. Consequently, the time of fungicide application can be delayed in resistant varieties. This will help to reduce the use of fungicides to the bare essentials as requested for the integrated crop protection management.     

Factors affecting the occurrence of gangrene (Phoma exigua) in potatoes

The presence in soil from Scotland and England of Phoma exigua f. sp. exigua and P. exigua f.sp. foveata, which cause gangrene, is confirmed by isolation, and it is established that infection of tubers occurs before lifting, and after lifting from soil adhering to tubers. The distribution of the disease is related to soil moisture, gangrene being most prevalent in tubers from the north-eastern counties of Scotland where the moisture content of arable soils remains high throughout the growing season. The incidence of the disease may be affected by the haulm, either as a source of infection—though not an important source—or by its effect on the maturity of the tubers. The incidence of gangrene is less where haulm destruction is rapid. The incidence of gangrene in a crop is not related to its incidence in the seed tubers planted and, unless infection is severe, the effect on yield is not serious. Symptoms of skin necrosis were associated with tubers from acid soils, infected with P. exigua f.sp. foveata and stored at low temperature.     

The effects of length of the growth period and of nutrition upon potato-tuber susceptibility to dry rot (Fusarium caeruleum)

Standardized inoculations of the potato varieties Catriona and Doon Star over 6 years have shown that shortening the growth period, either by deferring planting or by removing the haulm prematurely, may considerably reduce susceptibility of the tubers to dry rot (Fusarium caeruleum). Although these treatments reduce dry-matter content of the tubers, no direct relationship between this factor and susceptibility was established. High pre-maturity susceptibility was confirmed at the time of flowering; as the haulm matured susceptibility decreased: when the haulm was dead tubers were resistant. Within a fortnight after any time of premature haulm removal, tubers became completely resistant and showed a decrease in sucrose content but not in content of reducing sugars. It is suggested that susceptibility in the immature tuber is closely related to the content of sucrose, which accumulates because of rapid translocation from the vigorously growing haulm. After harvest, tuber susceptibility slowly increased during storage, with an increase mainly in reducing sugars, but these two factors were not directly related. An application of 6 cwt/acre (753 kg/ha) of 12:12:18 NPK fertilizer to Catriona significantly reduced liability to infection. Additional nitrogen (6 cwt/acre ammonium sulphate) raised tuber susceptibility to that in a non-fertilized plot: when sulphate of ammonia was the only treatment given, the tubers were significantly more susceptible than those receiving complete fertilizer.     

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.     

Chemical techniques for control of stem canker and black scurf (Rhizctonia solani) disease of potatoes

Thiabendazole, iprodione and benodanil were tested for control of stem canker and black scurf disease of potatoes using seed tubers with or without black scurf planted in soil infested or not with Rhizoctonia solani . Dormant seed tubers were treated with fungicide dusts, dips or sprays and before planting dusts were applied to soil or to sprouts on seed tubers.
Plants grown from seed tubers with black scurf had more stem canker, stolon infection and Thanatephorus cucumeris and progeny tubers had more black scurf than from seed without black scurf. Infesting soil with R. solani increased black scurf but not stem canker or stolon infection.
Fungicide treatment of seed tubers controlled all phases of the disease and sprout and soil treatments also had significant effects. Numbers of tubers in July were usually increased by treatment although by harvest many small tubers (>3 cm) had been resorbed. Fungicide treatment substantially increased yield only when shoot tips were pruned by R. solani .
Regression analyses showed that stolon pruning, Thanatephorus and black scurf were significantly related to stem canker, and numbers of tubers in July but not in September were inversely related to stolon pruning.
The significance of sources of inoculum and the effects of controlling disease on tuber production are discussed in relation to methods of fungicide use.     

EFFECT OF TETRACHLORONITROBENZENE ON THE SPROUTING AND CROPPING OF POTATO TUBERS

Potato tubers dusted with a preparation containing 3% tetrachloronitrobenzene (TCNB) and kept in a nearly closed container in the laboratory showed reduced sprouting. The reduction was greater the lower the temperature and the earlier the application of the dust; it was less marked with tubers lying on a moist substratum. After long exposure, and especially at higher temperatures, sprouts of treated tubers tended to show a 'witch's-broom' appearance. The repressive effect of TCNB on growth rate disappeared as soon as the tubers were brought into ordinary air. To a certain extent the action of TCNB was antagonized by treatment of the tubers with ethylene chlorhydrin.
Replicated field experiments over three seasons, with four potato varieties, gave a measure of the check to bulking of the crop which is brought about by winter storage of seed tubers in presence of TCNB. This check was eliminated by airing the tubers before planting, the time necessary for this depending on conditions. An airing period of about 6 weeks, in an unheated greenhouse (apart from sun- heat) was found to be sufficient for this purpose.
The speed of emergence of the variously treated batches was closely correlated with the yields produced, especially in the earlier liftings.
The average number of shoots per plant was significantly increased by the dust treatment and this was reflected in a higher seed/ware ratio in the mature or late-lifted crop.
There was no evidence that the delay in sprouting brought about by TCNB was due to an effect on the respiration rate of the dormant tubers.
The concentration of active principle was materially diminished in treated clamps after the period ( c. 5 months) of winter storage. Estimates were also made of the persistence of TCNB on exposed surfaces in still and in moving air.     

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.     

Life Histories and Fitness of Two Tuber Moth Species Feeding on Native Andean Potatoes

In the inter-Andean valleys of central Perú, two species of tuber moth, Phthorimaea operculella (Zeller) and Symmetrischema tangolias (Gyen), often occur simultaneously in stored potatoes. Traditional farming communities in the region produce a variety of native potatoes for local consumption. These include Solanum tuberosum subsp. andigena, the presumed predecessor of commercial potatoes, S. tuberosum subsp. tuberosum. In this study, we examined resistance against P. operculella in ten native Peruvian potato varieties (Casa blanca, Chispiadita, Madre de vaca, Mamaco negro, Misha, Chorisa, Mamaco rosado, Occa papa, Vacapa jayllo, and Yana tornasol). We also compared resistance in the first five of these varieties against S. tangolias. Varieties with pigmented periderms showed moderate resistance (30?C40% against P. operculella in Mamaco negro, Mamaco rosado, and Yana tornasol and 55% against S. tangolias in Mamaco negro). All the other varieties were susceptible to both moth species. Small tubers tended to be the most resistant to the attack by both moths; however, this was not related to the availability of food for developing larvae, since pupal weight and development time were unaffected by the size of tubers. Similar responses by the two moths to native potatoes indicate that tuber resistance could be used to control the complex of tuber moths that damage potatoes in the Andes. We suggest that native potatoes, which are often easily introgressed with commercial potatoes, are a potential source of resistance against tuber moths.     

Survival of the causal agents of 'early-dying disease' (Verticillium wilt) of potatoes

Although the isolation of Verticillium albo-atrum and V. dahliae from soil and dried moribund stems following infection of a potato crop proved extremely difficult, both fungi were equally capable of overwintering in these substrates and of inducing disease in a subsequently planted susceptible crop. In the absence of a susceptible crop some weed species became colonized. The two species, however, appeared to differ in their capacity for survival both beneath a monocotyledonous crop and within the potato tubers. Colonization of the roots of wheat, barley, oats, rye and maize was observed with V. dahliae but not with V. albo-atrum. The latter appeared to be capable of prolonged survival in the tubers, whereas V. dahliae did not remain viable in storage over winter. Consequently only tubers infected with V. albo-atrum produced infected plants. The presence of the fungi within the tubers affected neither dormancy nor the initial development of the sprouts. Some correlation was noted between tuber size, the percentage of tubers infected, the distribution of V. albo-atrum within the tubers and the development of disease in plants subsequently grown from these tubers.     

Genes tagging and molecular diversity of red rot susceptible/tolerant sugarcane hybrids using c-DNA and unigene derived markers

Sugarcane is an important international commodity as a valuable agricultural crop especially in tropical and subtropical countries. Two bulked DNA used to screen polymorphic primers from commercial hybrids (varieties) with moderately resistant and highly susceptible to red rot disease. Among 145 simple sequence repeat and unigene primers screened, 37 (25%) were found to be highly robust and polymorphic with Polymorphism Information Content values ranging from 0.50 to 1.00 with the mean value of 0.82. Among these microsatellites, twenty one were used in the study of genetic relationships and marker identification in sugarcane varieties for red rot resistance. A total of 105 polymorphic DNA bands were identified, with their fragment size ranging from 54 to 1,280 bp. Jaccard’s similarity coefficient value recorded between closely related hybrids was 0.986 while lowest coefficient value of 0.341 was detected with distantly related hybrids. The average similarity coefficient among these hybrids was 0.663. Cluster analysis resulted in a dendrogram with two major clusters separating the moderately resistant varieties from highly susceptible varieties. Three group specific fragments amplified by unigene Saccharum microsatellite primers viz; two markers UGSM316₈₅₀ and UGSM316₆₀ were closely associated with moderately resistant varieties by appearing bands in this region but the bands were absent in highly susceptible varieties. Similarly UGSM316₄₀₀ marker was tightly linked with highly susceptible varieties by amplifying uniformly in sugarcane varieties showing highly susceptible reaction to red rot but it was absent in moderately resistant varietal groups. Validation of red rot resistance/susceptibility associated markers on a group of different mapping populations for red rot resistant/susceptible traits is in progress.     

Experiments on the Spread of Potato Virus X Between Plants in Contact

Experiments on the spread of five strains of potato virus X were made with seven potato varieties and with tomato plants both under glass and in the field. Spread by leaf contact between healthy and infected plants was confirmed, and it was also found that spread could occur between plants whose only contact was below ground.
The rate of spread was much greater in tomato than in potato plants, and virulent strains of the virus, which achieve a high concentration in infected plants, spread more rapidly than avirulent strains. In only one experiment with potatoes did more than 10% of the healthy potato plants exposed to infection become infected during one season.
Datura stramonium and tomato plants became infected when growing in soil containing sap or residues from X -infected plants.
It was common in the field for potato plants whose foliage gave no reaction for virus X at the end of the season to yield a mixed progeny of healthy and infected tubers. Such infections are thought to result from underground spread.
Attempts to transmit virus X from infected to healthy potatoes by means of Rhizoctonia solani failed. No examples of infection were found except when healthy plants came into direct contact with sources of the virus.     

DRY-ROT DISEASE OF THE POTATO

A laboratory method is described in which a modified hypodermic syringe is used to inoculate potato tubers with Fusarium caeruleum. This injection method gives consistent results and permits reliable assessments of factors such as varietal susceptibility.
The occurrence of arrested lesions is noted.
Suspensions of very high spore concentration or of spores already germinated did not greatly increase the amount of dry rot above that given by the standard too spores per inoculation, provided that spore germination was adequate.
The size of tuber and site of inoculation were found to have a considerable effect upon the results of inoculation experiments, large tubers being more susceptible than small ones and the heel end being more susceptible than the rose end. The necessity for uniformity of material used in inoculation experiments is emphasized.