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to---20° F. To prevent the potatoes from freezing two stoves were put into the car and the temperature kept at 120° F. for a week until the weather moderated. Then the stoves were removed and the car started on its journey.

In the light of Bartholomew's experiments it is clear that the blackheart of these potatoes was due to the high temperature to which they had been subjected in the car.

However, it is equally clear that many of the tubers did not experience the high temperature indicated. This is shown by the fact that many escaped injury. A temperature of 120° F. maintained for a week would have ruined every tuber subjected to it.

The shipment of potatoes in stove-heated cars appears to be a rather common practice. Shippers should understand that it is attended with risk. Temperatures above 100° F. may be quite as injurious as a freezing temperature.

BLACKHEART PRODUCED AT HIGH TEMPERATURES VS. BLACKHEART PRODUCED BY EXCLUSION

OF THE AIR.

Blackheart may be produced in different ways by varying the three factors involved, viz., the temperature, the oxygen supply and the length of the exposure.

In this study the writers have focused their attention upon the oxygen factor and employed low temperatures (below 75° F.) in most of their experiments. Only one series of experiments was made with tubers in an incubator at a temperature above 100° F. The blackheart so produced (Plate XII) was like that described by Bartholomew and resembled the blackheart produced in our own experiments by exclusion of the air at lower temperatures.

However, there appears to be one character by means of which it is possible to determine whether a given case of blackheart was brought about by exposure to a high temperature for a short time in the presence of an abundant supply of air, or by the exclusion of the air for a long time at a low temperature. Tubers affected with blackheart produced by high temperature usually appear normal externally, 12 while those affected with blackheart produced by exclusion of the air usually show more or less surface discoloration. (Plate I.)

12 Bartholomew, loc. cit., p. 637.

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RELATION OF BLACKHEART TO SEED POTATOES.

When seed potatoes show blackheart the question arises, How does it affect their value for planting? Also, is it safe to plant unaffected tubers from the same lot?

Comparative planting tests have not been made, but observations on the sprouting of tubers affected with blackheart leads to the belief that only severely affected tubers need be rejected. The sprouts of blackhearted potatoes appear to be normally robust and vigorous.

If the blackheart is accompanied by the discoloration of a considerable portion of the surface the affected tuburs should not be used for planting. Eyes on the discolored areas will not sprout and such tubers are liable to decay quickly. Probably, it is advisable to reject, also, all tubers having large areas of blackheart, even tho normal externally. But tubers free from external discoloration and showing only small to medium-sized areas of blackheart may be planted. As for associated tubers which show neither blackheart nor external blemishes, there is no reason to suspect that they are in any way injured for seed purposes. If seed potatoes are sound and normal in appearance it is unlikely that they have been injured by any condition of storage to which they may have been subjected.

APPLICATION OF THE RESULTS TO THE PROBLEM OF

POTATO STORAGE.

The results of this investigation emphasize the importance of providing ventilation for potatoes in storage. The need of ventilation depends very largely upon the temperature. As the temperature rises the volume of air required increases rapidly. At low temperatures, potatoes may be stored in deep piles for long periods of time. At high temperatures, it is necessary to avoid deep piling or else provide special means of ventilation. If the temperature is kept below 40° F. potatoes may be piled in bins and cellars to a depth of six feet without any ventilation except that provided thru free access to the air overhead. Under such conditions, potatoes may be stored with safety for at least six months and perhaps longer. It is probable that no harm will result if the temperature goes up to 45° F. for a few days. But a long period of storage followed by a two weeks' exposure to a temperature of 50° F. or higher is likely to

result in the ruin of most of the tubers below a depth of about three feet. Potatoes stored in deep piles should be carefully watched in the spring as the temperature rises. A few days of high temperature may cause much loss.

It is not in the province of this bulletin to discuss the means by which the ventilation of potatoes may be secured. The methods to be employed will vary according to circumstances. The principles to be kept in mind are as follows:

(1) Potatoes stored at high temperatures require more ventilation than those stored at low temperatures.

(2) Better ventilation is required for potatoes which are to be stored for a long period than for those which are to be stored for only a short time.

(3) Until more accurate determinations are made, six feet should be regarded as the maximum depth to which potatoes may be piled without special provision for ventilation when stored for six months at temperatures below 45° F. If greater depths than six feet are employed ventilators should be provided and so arranged that none of the tubers will be more than six feet distant from an abundant supply of air.

(4) At temperatures of 50-70° F. potatoes should not be piled over three feet deep if they are to be kept longer than about three weeks.

(5) No kind of ventilation is sufficient to prevent the occurrence of blackheart in potatoes kept for even a few days continuously at a temperature above 100° F.

(6) Complete exclusion of the air will ruin potatoes at any temperature.

(7) Small potato pits do not need ventilation; but some provision should be made for the ventilation of large pits.

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NECK-ROT DISEASE OF ONIONS.'*

M. T. MUNN.2

SUMMARY

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The neck-rot disease of onions apparently occurs in every oniongrowing district in Michigan and New York and in every principal onion section in the United States. It causes heavy losses to stored onions and frequently to the growing crop and the seed crop. Evidence has been secured that the disease is destructive in Europe. The most striking signs of the disease are to be found in the storage houses where the causal fungus appears as masses of sclerotia at the necks of the onions. However, these may cover part, or all, of the individual bulbs. The smoke gray covering of fungous growth commonly recognized by onion growers as is another striking sign of the disease.

The cause of the disease as determined by numerous inoculation experiments is a fungus which has been described and given the name Botrytis allii. Infection of the bulbs in the field occurs thru the leaves and necks. The fungus passes down the necks and causes a neck rot in the field, or else it goes into the storage house in the form of mycelium in the necks and bulbs where the disease develops rapidly under favorable conditions. The bulbs may also become infected from the soil. The seed

1 Also presented to the faculty of the Michigan Agricultural College in partial fulfilment of the requirements for the degree of Master of Science.

2 Acknowledgments are due Mr. F. C. Stewart, Botanist of this Station, and to Drs. Ernst A. Bessey and G. H. Coons of the Michigan Agricultural College for valuable aid in many ways during this study. Thanks are also due Professor H. H. Whetzel of Cornell University who read the portion of the manuscript relating to nomenclature and who, because of his knowledge of the genus Botrytis, was able to make many valuable suggestions. A number of prominent onion growers have made this work possible by placing their onion storage houses and onion fields at the disposal of the writer for the purpose of experimentation and observation. Special mention should be given the names of Messrs. C. C. Mowris and Henry Greffrath of South Lima, N. Y., L. J. De Martinas of West Bloom field, O. L. Humphrey of Honeoye Falls, and the J. B. Frey Co. of Rochester, N. Y. The onion growers of Michigan, also, responded liberally in many particulars. Messrs. E. F. Clark & Son of Wayland, Michigan, deserve many thanks. Reprint of Bulletin No. 437, July, 1917.

(319)

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heads become infected by wind-blown spores, causing a blasting of the flowers.

The fungus apparently produces appreciable amounts of oxalic acid. However, it was not possible to demonstrate that this acid produces the pathological changes in tissue observed when the fungus attacked the host. It was found that the fungus secretes the enzym pectinase. This enzym was precipitated and re-dissolved from the fungal extract. From a study of the action of the fungal extract upon plant tissue discs it was possible to explain all of the tissue changes taking place when the parasite attacks the host.

Factors favorable to infection and the occurrence of the neck rot were found to be as follows:- (1) Immaturity and imperfect curing of the bulbs; (2) the application of commercial fertilizers late in the season or in incorrect proportions; (3) the application of large quantities of stable manure before planting; (4) poor air drainage in the onion fields; (5) high humidity, high temperature and poor ventilation in the storage house.

Methods of control, so far as worked out, lie mainly along the line of field sanitation, care of the curing crop, and storing in properly constructed and regulated houses. Diseased bulbs should not be used as mother bulbs " for producing the seed crop since they are an important source of infection to the seed heads, and later, to the growing crop in nearby fields. Piles of decaying and refuse onions should not be permitted near the onion fields or used for fertilizer thereon. Care should be exercised in handling the crop to prevent unnecessary bruising on the topping machine and while handling the stock to be stored. The crop should mature and die early. Remove all refuse tops, soil and screenings. Cure thoroly in the field or in the drying sheds. Store the crop in slatted crates in well constructed houses having ample facilities for ventilation and the maintenance of low temperature.

Fumigation of the stock with formaldehyde gas either just before or after storing has been found to be both ineffective against the fungus and impracticable in the average storage house. While spraying the growing crop with bordeaux mixture has given some promising results, it has not been tested sufficiently to warrant definite recommendations. Apparently, the spraying of onions has not proven popular with onion growers because of certain difficulties encountered.

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