THE DISEASE AS FOUND IN THE FIELD. THE LEAF-SPOT DISEASE OF TOMATO BY G. H. COONS AND EZRA LEVIN This disease caused a 'practical failure of the tomato crop in 1915. It may he expected to he serious whenever the season is moderately wet. It is probably the worst disease of the tomato. This bulletin discusses the cause of the disease and gives practical recommendations IAL BULLETIN No. 81. IKE LUMMY OF THE IJRIYERIITY OF IIUOMS JUNE. 1917 Michigan Agricultural College EXPERIMENT STATION BOTANICAL DEPARTMENT for control. ^ * EAST LANSING 1917 Digitized by the Internet Archive in 2017 with funding from University of Illinois Urbana-Champaign Alternates https://archive.org/details/leafspotdiseaseoOOcoon 551. Z C.'JZt c^p.3 CONCISE DIRECTIONS FOR CONTROL. It is definitely known that seedlings commonly carry the The disease infection to the field. The seed is not thought to carry the to theHeid fungous spores. Therefore, if the soil in the flats or cold seedlings, frame is clean the plants start clean.* * To insure clean seedlings spray the young plants at least sjvray young twice with weak Bordeaux mixture (2-2-50). t Do not follow tomatoes with tomatoes. Have a system- The disease is atic rotation for your fields. Cleaning up of trash in garden Sverfrom hve patches and fields is a good sanitary measure. Do not throw year old vines on the manure pile or compost heap. diseased Where the type of culture permits, staking of tomatoes The spores will largely prevent damage from tomato leaf -spot. Staked the a soii f and tomatoes give greater yields, earlier crops and allow most i^r leaves, efficient spraying. After the plants are well started in the field, spray every be 10 days with Bordeaux mixture lf-^-50, made exactly accord- protected by ing to directions. Tlie early sprayings are most important m?xture! x in preventing the disease from getting a foothold. When the plants are large, unless the application is thorough it will pay to spray down and back on the same row. This, if a gentle wind is blowing and if the sprayer gives a good * mist, will insure a thorough application. Tomato leaves make a quick growth. Frequent sprayings payings are * are necessary to keep the new growth covered. to keep the new growth covered. *(1) Soil from fields which have not recently grown tomatoes is safe for use. Manure containing tomato trash is unsafe. (2) Used soil may be sterilized by steaming by the inverted pan method. (3) Used soil may be disinfected by drenching* with formaldehyde 1 to 100 parts water. Use 1 gallon to the cubic foot, drenching the soil completely. A quart of solution will disinfect a flat. Cover with paper or canvas over night. Then turn the soil thoroughly to allow the fumes to escape. As soon as the soil is in good condition, plantings may be made. This treatment is simple, cheap and easy. It will also prevent damping off. 52149 Fig. 2. Diseased plant showing the killing of the lower leaves. INTRODUCTION* Michigan is one of the great tomato-growing states. The Michigan crop produced amounts to nearly a million dollars annually. ^ worth r ° P Tomatoes are grown for ordinary market purposes, for can- annually 30 ning and for seed. The type of culture and the varieties used vary with the utilization of the crop. The methods used in growing the crop influence the prevalence of this disease to a great extent. For example, it is found that where tomatoes are staked, the damage from leaf-spot is greatly reduced, in general, seedlings are grown in green- houses or cold frames and set by hand in the fields. This method of procedure lends itself readily to control measures which entirely prevent damage from the leaf-spot disease. THE IMPORTANCE OF THE LEAF-SrOT DISEASE. The leaf-spot disease of tomato, commonly called by grow- The leaf spot ers “blight,” caused a practical failure of the crop in many slnous™ 081 localities in 1915, and for the preceding five years, has ma- ^ease^of^ terially shortened each growing season. This condition is found in every state where tomatoes are grown commercially. The consensus of opinion of pathologists from all parts of the United States, is that the Septoria leaf-spot is the most serious disease of the tomato. The disease has been found in the northern, as well as the The disease southern parts of Michigan. Its distribution is probably as spread as ~to- great as tomato culture. mat0 culture. HOW TO TELL THE DISEASE. The signs of the leaf-spot disease are well expressed by the Leaf spot common name “blight.” The growers notice a general un- bUghting of a thriftiness in the plants along with a blight which begins on {J^the 8 the lower leaves and which gradually advances upward on bottom the plant. (Fig. 2.) Blighted leaves if carefully examined up ' vard are seen to be covered with small circular spots, commonly less than % of an inch in diameter. In some cases a whole leaflet may be involved. These spots are sometimes black, but are usually grayish with a black border. (Frontispiece.) As the disease progresses the spots dry and the leaflets turn *This popular bulletin is based on Technical Bulletin 25 of this Station. Anyone interested in the more detailed account may secure the technical bulletin so long as the supply lasts. Those who make request, may be placed upon the mailing list to receive bulletins from the College. 6 EXPERIMENT STATION BULLETIN. Spots appear on the leaf- lets. Then the leaves die, falling from the plant at the least jar. The leaves manufacture sugar and starch. The leaf spot disease destroys the factories. yellow. The leaves die and drop from the plant at the slightest jar. The disease advances up the plant, and soon all leaves, except a small tuft at the end fall from the stem. The dropping of the leaves exposes the fruit, which may sunburn badly. Spots are also formed on the calyx and stem but rarely on the fruit. The damage to the crop comes largely from the loss of the leaf surface. The leaves of plants are the manufacturing organs which produce the starch and sugars which make up the greater part of the solid matter of the fruit. A leaf dis- ease interferes with the manufacturing power, hence, the Fig. 3. Diseased spots magnified five times. Small, sour, watery tomaloes are caused by leaf spot. fruit grows slowly or not at all. Half-matured fruit fails to ripen. The crop from plants with blighted foliage is small, sour and watery. This last condition is often com- plained of by canners ; although they ascribe “watery” tomatoes to a wet season, rather than to a fungous disease made severe by the season. In short, all the general signs are those which go with disturbance of food manufacture by the leaves. THE CAUSE OF LEAF-SPOT. a fungus Leaf-spot of tomato is caused by a parasitic fungus called thlftomato Septoria ly coper sici which grows and feeds in the tomato plant. leaf. A parasitic fungus is a microscopic plant which makes THE LEAF-SPOT DISEASE OF TOMATO. 7 no food for itself but which steals its living from another plant. If a diseased spot is examined with a microscope, the leaf tissue is found to be pierced by minute threads. These threads are the body of the fungous parasite. After the threads have gro,wn in the leaf and occupied considerable area, small black dots appear on both the upper and lower surfaces. (Fig. 3.) The small black dots are hollow spherical cases Needie- whicli contain countless long needle-shaped bodies called areffe spores spores. (Fig. 4.) These spores are to the fungus what seeds are to the higher plants. A spore washed to a tomato parasitic leaf and given favorable temperature and moisture will fungus ' Fig. 4. Thin section of spore case, showing long needle-shaped spores. Magnified 800 times. sprout and enter the leaf, quite as a seed sprouts and grows, only the soil the fungus uses is the leaf tissue. The spores of this fungus have a jelly-like coat and when The spores a diseased leaf is wet, the spores swell and push out of the on a wet bout spore cases in sticky masses. When the spores are once out- leaves * or side the spore case they float apart in the water on the leaf and frequently sprout and enter, thus making the spot larger or causing new ones. The spores from a diseased leaf may be washed or splashed Rains splash from leaf to leaf during rains, or if the plants are worked about* 0168 when wet, may be carried from plant to plant on the culti- vator or on the clothes of the worker. The spores can endure at least three days drying, hence, cuiuvatmg those which wash to the ground may be dried on the dust spreads^he and blown about, thus leading to a wider distribution of the p^ant s to rom disease in the field or from field to field. plant. 8 EXPERIMENT STATION BULLETIN. How the spore attacks the tomato leaf. In 13 days one spore produces thousands of similar spores. Either the upper or lower surface may be the point of entry. The fungus lives over on old trash from last year’s diseased plants. The weather does not cause the leaf spot disease. If a spore is put under proper conditions it will begin to sprout in about twenty -four hours. The sprouts are threads which push out from the needle-like spore. In a few hours after sprouting the thread-like tube enters the breathing pore of the leaf. Then the fungus grows in the tissues of the plant, twining among the cells. In about six days after the spore reaches the leaf, the first watery discoloration produced by the fungus can be seen. In less than ten days the spot is definite and the killing of the invaded leaf tissue is com- pleted. The fungus fruits in about 13 days and produces several black spore cases. Hence, one spore gives rise to countless thousands of other spores, each capable of repeat- ing this story. If the lower surface is the point of entry the spots are large and may involve one-half or the whole of a leaflet. If the upper surface is the point of entry the spots may be no larger than the head of a pin. Usually the upper leaves show this latter type of infection, evidently from dust as a source. The lower leaves are infected from beneath, doubtless by splashings from the ground or from the older leaves. The fungus lives over winter on the trash from a preced- ing tomato crop. It lives over in the greenhouses or cold frame in the trash from seedlings left in the soil. The spores have not been found on tomato seed. Hundreds of plants from all the common varieties have been grown in clean soil without one case of leaf-spot appearing spontaneously. The spores of the fungus are released from the spore case only when the leaf is wet. Heavy dews give conditions which allow an oozing of spores. By far the greatest factor in spreading the fungus is a washing, splashing rain. There- fore, the disease is most serious in a rainy season such as that of 1915. Many growers make the mistake in thinking that the tomato disease is caused by the weather, while the truth is that the disease is caused by a parasite whose spread is favored by the wet conditions. Growers must adopt measures which will insure a crop in spite of the weather. THE LEAF-SPOT DISEASE OF TOMATO. 9 HOW TO PREVENT LOSS FROM TOMATO LEAF-SPOT. Resistant Varieties. In an experiment with more than 50 of the leading tomato varieties, no variety was found strikingly resistant or sus- ceptible. Many varieties are vigorous growers and continu- ally keep ahead of the leaf -spot by the production of new foliage. Dwarf varieties usually suffer more damage than the ordinary sorts. Before other control measures can be considered, we must recognize the following observations that have been dealt with above: 1. The sources of infection are at least two: the green- house or the hotbed and the diseased trash in the field. 2. Infection results from inoculation upon the upper and lower surfaces of the leaf. 3. The period from the time of inoculation to spore exuda- tion is about 13 days. 4. Moisture is the primary agent in dissemination of the disease. 5. The exudate of spores is in the form of a mucilaginous mass. The spores are always transferred by some external agency. 6. It has been shown that the fungus will not grow at 85° F. and will not revive after 5 days at 98° F. Since such temperatures are frequently reached during the summer months, the heat relation must be taken into consideration as a natural means of checking the disease. The most important control measures for this disease are preventive. Clean seedlings in clean soil, if reasonably isolated remain practically free from the disease. In order to be sure that the seedlings do not become dis- eased at the start, clean soil should be used in the green- houses devoted to seedling production. The soil should be fresh or sterilized. The seedlings should be sprayed as soon as their height above ground makes it practicable, and again before being transplanted to the field. For this a weak Bordeaux mixture (2-2-50) is advised. Since it has been determined that wintered-over, diseased vines possess spores which are viable, the old trash must be destroyed as far as possible. Since, however, this is not practical except in greenhouses and gardens, rotation is strongly urged. While there is no experimental evidence to demonstrate the value of rotation as a means of control, numerous instances have been noted in which rotation has been successful in controlling the disease. No resistant variety is as yet found. Summary of facts about the disease. Prevention methods necessary. Clean soil. Destroy old trash. 10 EXPERIMENT STATION BULLETIN. The spores are carried when wet. Spray with Bordeaux. Reasons for failures in control. How to spray. Spraying has succeeded in in Michigan. Since spores are released from the spore cases when wet the plants should not be “worked” until the plants are dry. Growers have reported cases where cultivation began at a small infected patch and the disease was carried over the entire field, and that in less than three weeks the entire field was spotted. This is now readily explained. In green- house practice, irrigation should replace the ordinary use of the hose. After transplanting to the field, spray with 4-4-50 Bordeaux mixture every 10 days. As has been shown, the period from time of inoculation to spore exudation is at least 13 days. Allowing this leeway for differences in period of infection, it would seem that a spray so applied would give the necessary protection. The greatest part of and the strongest infection results from inoculation on the lower surface of the leaf. There- fore, all the precautions are less effective if the plants are allowed to run at will over the ground. The spraying of the under side of the leaf is not accomplished unless the appli- cation is thorough. Failures to control the disease by spray- ing are doubtless due to lack of thoroughness and timeliness. Spraying with 4-4-50 Bordeaux mixture in the fields where tomatoes are staked should be extremely successful. In this state spraying will not be most effective unless the spray reaches the lower surface of the leaves. A sprayer such as is used for potatoes, with two side nozzles set to shoot upward and if practical, with one central nozzle to spray downward for each row will, under high pressure, be most efficient in Michigan fields. Spraying tomatoes with Bordeaux is a common practice in many tomato-growing centers. Wherever it has been tried it has become a fixed part of the culture of the crop. No grower can afford to risk his crop, and no company can afford to risk its acreage. The spraying experiment of the College in 1913 at Plymouth is a good example of what may be accomplished. In this experiment “the plants that were sprayed with Bordeaux mixture four times, produced 12.5 tons per acre while the plants that were not sprayed pro- duced 5.2 tons per acre. The first spraying was made about one month after the plants were set in the field and fre- quently enough afterward to protect the new growth.” (Every ten days or two weeks).* Copper sulphate (Bluestone or Blue Vitriol) is a little more expensive this year than formerly. In spite of the cost, spraying is a crop insurance which is most strongly to be advised. Eustace, H. J., Mich. State Board of Agriculture Report 1914 :234. THE LEAF-SPOT DISEASE OF TOMATO. 11 THE SPRAYER. Tlie type of sprayer depends upon the acreage and method of culture. The market gardener can control tomato leaf- spot with a small knapsack or hand sprayer. Tomatoes are commonly grown in fields of 5 to 10 acres. For fields of this type a horse-drawn traction sprayer is to be advised. However, efficient work can be secured by means of a handpower barrel pump mounted on a truck. This is the cheapest form suitable for field conditions and may be rigged up for from $25 to $40. For large acreages or for community or company use, a power sprayer is the best investment. Such an outfit will put the Bordeaux on most efficiently and will give the maximum protection. Knapsack sprayer for the small grower. Horse-drawn traction sprayer for the ordinary field. The sprayer for large acreages. 12 EXPERIMENT STATION BULLETIN. ARRANGEMENT OF NOZZLES. Arrange the The necessity of spraying the under side of the leaves spray 6 upward, makes the arrangement of nozzles important. The accom- panying diagram shows the arrangement believed to be most efficient. Fig. 5. Arrangement of nozzles. With such a setting of nozzles the leaves can be efficiently covered without repeating the spray in the reverse direc- tion. Any spraying outfit can be adjusted to suit the needs of the crop, by use of a few pipe fittings. The accompanying diagram suggests a cheap, easily arranged setting. (Fig. 5.) hesf y oii can ^l )ra yi n & worth while even if the outfit is not ideal, or Abetter than the nozzle arrangement perfect. Given good Bordeaux, none at all. thoroughness of application with pressure enough to produce a mist, and the investment will yield good returns. THE LEAF-SPOT DISEASE OF TOMATO. 13 THE MAKING OF BORDEAUX. FOR SMALL ACREAGES. (1) Saw a 50-gallon barrel (vinegar or oil) and make two 25-gallon tubs. (2) Put 2 pounds of Bluestone (called Blue Vitriol or copper sulphate) in a cloth sack and hang over night in one- half tub of water (12% gallons). Bluestone dissolves slowly. Hang it the night before so that the sack is just under the surface of the water. DIP EQUAL PARTS FROM 1 AND Z INTO 3 Fine mesh screen and funnel to strain Bordeaux Use this mixture at once in sprayer Dprayer Fig. 6. How to make Bordeaux mixture for small acreages. (3) Make a lime paste by slaking 2 pounds of fresh stone lime in one-half pail of water. Stir this into the half tub (12% gallons) of cold water. Hydrated lime (comes in sacks) may be used. Use 2% to 3 pounds of hydrated. Then follow the directions in the picture. This method will make 25 gallons of spray. 14 EXPERIMENT STATION BULLETIN. FOR LARGE ACREAGES. To make 100-gallon batches. Bordeaux 4-4-50 or 8 - 8-100 formula. (1) Secure four 50-gallon barrels. (2) Fill barrel No. 1 half-full, and hang 25 pounds of Bluestone so that the Bluestone is just under the surface of the water. This makes stock Bluestone, 1 pound to the gallon. (3) Fill barrel No. 4 half-full of water and stir 25 pounds of freshly slaked stone lime or about 30 pounds hydrated lime in it. This is the stock lime paste, about 1 pound to the gallon. The stock solutions will make 300 gallons of spray. They will keep if covered. (4) Take 8 gallons stock Bluestone from barrel No. 1 and add to 42 gallons of water in barrel No. 3. (That is, till the barrel) . 1 25 LB£ BLUE-\ STONE IN 25 \ GALSL WATER!) 25LB5.ST0N£\ LIME IN 25 ,|L ^ALS/WATERiT I DILUTE |li [BLUESTONE LUTE MI LI 0F;LlMEi ADO 8 GALS. OF STOCK BLUESTONE FROM 0bl. No.1To4£ GALS. OF WATER. ADD 8 GALS. OF STOCK LIME FROM Obi. No.4TO 4 2. GALS. OF WATER. Fig. 7. How to make Bordeaux mixture in 100 gallon amounts. THE LEAF-SPOT DISEASE OF TOMATO. 15 (5) Take 8 gallons of stock lime from barrel No. 4 and acid to 42 gallons of water in barrel No. 3. (6) Run these materials together through a strainer into the sprayer tank. The combination is Bordeaux mixture and must be applied at once. If the spray tank holds only 50 gallons make the dilution on one-half the above scale. The first batch may be tested with litmus paper to make sure that there is lime enough. If blue litmus paper turns red, add more lime. With good fresh lime using the amounts given, no test is needed. If weather conditions prevent the using of a batch of Bordeaux, it may be kept for a long time if cane sugar is added at the rate of one-half pound to 50 gallons.