\m 23 5 94 2 1 'Introduction' \m 47 14 61 2 1 'Glume Blotch' \m 149 60 22 4 1 'Helminthosporium Leaf Spot' \m 167 66 87 4 1 'Leaf Rust' \m 192 76 41 5 1 'Powdery Mildew' \m 202 79 4 5 1 'Stem Rust' \m 219 84 20 5 1 'Fungicide Control Summary' \t'February 1983' 'IV L-'page Dr. Tom Kucharek, Extension Plant Pathologist DISEASE CONTROL IN WHEAT Plant Protection Pointer No. 27 Control of Glume Blotch, Helminthosporium Leaf Spot and Leaf Rust of Wheat Using Fungicides Applied by Aircraft \h 1 'Introduction' Introduction: Glume blotch, Helminthosporium leaf spot, leaf rust and powdery mildew are fungal diseases that reduce wheat yields and quality. Ideally the use of resistant varieties would be the best methods of controlling these plant diseases but such is not always available. Currently, a grower can purchase varieties with disease resistance to both leaf rust and powdery mildew and it is strongly advised he do so. Resistance to glume blotch and Helminthosporium leaf spot is lacking. TO MAXIMIZE WHEAT YIELDS the grower can use varieties with resistance to powdery mildew and leaf rust and couple this with aerial applications of mancozeb (Dithane M-45 or Manzate 200) fungicide to counter the devastating effects of glume blotch and Helminthosporium leaf spot. Should the grower plant a leaf rust susceptible variety, aerial applications with mancozeb will reduce this problem also. UNDER NO CIRCUMSTANCES is this spray program with mancozeb designed to control powdery mildew. Bayleton is cleared for use on wheat and it is effective against powdery mildew. \h 1 'Glume Blotch' Glume Blotch: GLUME BLOTCH IS CAUSED BY the fungus Septoria nodorum. It has been epidemic on wheat in many Florida fields in most of the last 13 years. The reason for this disease being somewhat inconsistent in occurrence is due to three major factors. 1) This fungus can be seedborne but apparantly not all seed lots are infected. No seed certification program for diseases is available to eliminate the guess work for the grower. 2) This fungus can live on old wheat stubble from one year to the next and even to some degree for two years. Crop rotation with wheat not being planted on the same piece of land for at least three years would be adequate to reduce glume blotch inoculum below levels necessary for epidemic development provided that seed were not infected at planting time. \v \b 3) Because temperatures favorable for foliar diseases of wheat exist each season, rainfall is a major influence on the severity of glume blotch. Spores are produced abundantly between 64 and 77 F but are produced to some degree above and below this range. Spore germination and infection occurs between 50 and 86 F. Symptoms occur between 8-14 days after infection when temperatures are 64 to 86 F. These temperatures occur commonly during spring months in Florida where wheat is grown and thus temperature does not appear to be a limiting factor for disease development. Also, such temperatures are common in October and November, a period of time when some growers plant wheat. "Warm winters" (December, January, February and March) are conducive for epidemic development as conditions will be suitable for earlier disease development which in turn will result in more damage. GLUME BLOTCH IS DEPENDENT ON rain and leaf wetness for spread, infection and symptom development. Thus wet seasons are conducive for glume blotch epidemics. Warm and wet winters will allow for an earlier development of glume blotch in the spring. Planting wheat after November 15 will reduce inoculum buildup of glume blotch in the fall thereby delaying the epidemic in the spring and such later plantings will not be detrimental to yield. Glume blotch begins on lower leaves before stem elongation occurs. As the plant matures glume blotch progresses upward on successive leaves formed during stem elongation. Should glume blotch or other diseases progress onto the flag leaf (last leaf formed which is immediately below the spike), significant yield loss will occur because the flag leaf is a major contributor of food products for seed development. USE OF MANCOZEB fungicide to control glume blotch on the flag leaf and spike is profitable for the grower if the spray program begins prior to full emergence of the flag leaf. When the spray program is initiated at growth stage 7 (second node of stem visible) or 8 (last leaf just visible), glume blotch is reduced on the lower part of the plant thereby reducing inoculum (spore) supply that could be a source of infection for the flag leaf when it is formed. Repeat applications at 10-14 day intervals are for the purpose of replenishing a declining fungicide residue on previously sprayed leaves and depositing a residue on newly formed leaf tissue, especially the flag leaf. Because a limit of 3 applications of mancozeb are legal for use on wheat and because residues of this fungicide decline with time, the grower should not begin the spray program prior to growth stage 7. To do so would reduce effective residues later in the season if three applications are made on 10-14 day intervals. To begin the spray program prior to growth stage 7 and lengthen intervals are equally counter productive. The best compromise for spray scheduling at this time is to aerially apply mancozeb at growth stage 7 (second node of stem visible) REGARDLESS OF MOISTURE CONDITIONS and apply the next two applications at 10-14 day intervals IF wet weather begins or persists. Use the 10-14 day interval to your advantage by applying the fungicide JUST PRIOR TO WEATHER FRONTAL PASSAGE should one be forecasted during the 10-14 day period. Should frontal passage \bnot occur during this period but glume blotch has been observed in the planting and wet conditions are anticipated, extend the spray interval to 14 days. If extremely dry weather persists at this time, the grower should consider terminating the spray program. The above mentioned options may sound too "iffy". But, when this approach is used over a period of years, the full season spray programs by the grower during severe "disease years" is expected to offset the minimal cost incurred in those years when the grower applies only the first critical spray but does not need to second two applications. In other words, scheduled sprays 2 and 3 will not return a profit if scheduled spray 1 is not applied at the right time. LEAF SYMTPOMS OF GLUME BLOTCH begin as tiny dark specks which expand in size up to 1/4 inch or more and being shaped like a football or somewhat elongate. The spot may be tan or brown to black in color with or without a yellow halo. Small black pimple-like structures may be seen in the spots especially on the undersides of the leaf. On the heads, glume blotch produces an off color (grey to brown) usually on the upper half of the individual glume (outer most cover of flower or seed). The black pimple-like structures (pycnidia) form abundantly on glumes. This fungus can oversummer on or in seed and wheat debris in the soil, thus providing inoculum (spores) for the following season. Spores are produced within a black, somewhat round vesicle (pycnidium) that is partially submerged in the wheat debris or living plant tissue. \h 1 'Helminthosporium Leaf Spot' Helminthosporium Leaf Spot: Helminthosporium leaf spot is caused by the fungus Helminthosporium sativum. It is similar to glume blotch in appearance but does not produce pycnidia. Rather, it produces masses of green to brown spores near the center of the spot which will have an olive color when viewed with a hand lens. Like Septoria nodorum, this fungus can be carried on or in the seed or it may survive in the field or organic debris. Warm and wet weather is conducive for this disease. The spores of H. sativum are distributed primarily by wind. Control of Helminthosporium leaf spot should be done in the same manner as previously described for glume blotch. Helminthosporium leaf spot can occur on the same leaf with glume blotch or by itself throughout the field. \h 1 'Leaf Rust' Leaf rust: Leaf rust is primarily a leaf disease that occurs during the spring months on wheat in Florida. It is caused by the fungus Puccinia recondita f. sp. tritici. Spore germination can occur from 36 to 90 F with optimum temperatures being 50 to 81 F. Optimum temperatures for infection are 65 to 77 F. Time from infection to symptom expression may be as long as 21 days with temperatures of 46 F or as short as 8 days with temperatures of 74 F. Optimum temperatures for epidemic development occurs commonly in the spring months in North Florida. \v \bLeaf rust appears as orange to brown pustules up to 1/16 inch across. Leaf rust pustules are found on leaf blades first and later on leaf sheaths, peduncles and occasionally on the heads. Rubbing a white cloth across a leaf will result in an orange to brown dust deposit on the cloth. Leaf rust is best controlled with resistant varieties. At this time ample resistance is available in the agronomically acceptable varieties, Florida 301, Florida 302, Coker 762 and Coker 797. Should you plant a leaf rust susceptible variety, the spray program will reduce leaf rust if the same spray scheduling for glume blotch is used. \h 1 'Powdery Mildew' Powdery Mildew: Powdery mildew, caused by the fungus Erysiphe graminis tritici, forms a white powdery growth on leaves. This disease must be controlled with resistant varieties such as Florida 301, Coker 762 and Coker 797. Use of a fungicide spray program for powdery mildew in Florida lacks supporting data. \h 1 'Stem Rust' Stem Rust: Stem rust, caused by the fungus Puccinia graminis tritici, is found occasionally when susceptible varieties such as McNair 701 are planted. Stem rust appears on leaves and stems as orange to red pustules. They are generally more elongate than pustules of leaf rust and are common on stem tissue as long pustules up to 3/8 inch. Stem rust pustules are a darker red than leaf rust pustules. Stem rust pustules often have remnants of ruptured leaf tissue (epidermis) around the pustule whereas leaf rust does not. Resistant varieites are the only means by which stem rust should be controlled in Florida. \h 1 'Fungicide Control Summary' Fungicide Control Summary: Use of fungicides to control foliar diseases of wheat in Florida is directed at 1) glume blotch, 2) spot blotch and 3) leaf rust. The leaf phase of stem rust will also be reduced if stem rust should occur. The key to using fungicides profitably on wheat is to begin spray applications early (growth stages 7 (second node of stem visible) to 8 (last leaf just visible)). BEGINNING SPRAY APPLICATIONS LATER WILL NOT RETURN A PROFIT. \bKey points are: 1. Use Dithane M-45 or Manzate 200 fungicide. 2. Use either fungicide at 2 lbs/application/acre. 3. Aerial applicator should use 5 gallons water/acre as a fungicide carrier. 4. AERIAL APPLICATORS SHOULD ADD A SPREADER-STICKER TO THE TANK MIX. 5. Use 3 applications per season per field.* IMPORTANT: 6. Apply first application when wheat is in growth stage 7 or 8. Beginning later reduces benefits. The second and third applications should be made 10 to 14 days after the prior application. If the weather forecast shows a front with rains on its way and it will interfere with 10-14 day interval, spray just BEFORE front moves in. *Glume blotch, Helminthosporium leaf spot, and leaf rust will be more severe in "wet" years. Because of limited weather forecasting capabilities, and because this spray program is not expected to return a profit in "dry" years, the grower is advised to apply the first spray at growth stage 7 or 8. Should dry weather ensue, further spraying may not be necessary. Should rains occur then continue the spray program. Remember, sprays 2 and 3 are not of value without spray 1 applied at growth stages 7 or 8.