Identify conditions that increase disease pressure in your field.
Corn Disease Models
Two diseases are summarized under the name Corn Leaf Blight. In the moderate and cool climate we will find the Northern Corn Leaf Blight caused by the disease Helminthosporium turcicum. Whereas it warmer climate we might find the Southern Corn Leaf Blight caused by the pathogen Bipolaris maydis (Nisikado & Miyake) Shoemaker, teleomorph Cochliobolus heterostrophus (Drechs.) Drechs. This disease became famous when it has led to very high losses in the year 1970 in which a very warm summer climate combined with a very successful parent genome widespread in the corn grown in the US at this time.
Southern Corn Leaf Blight is a disease caused by the pathogenic fungus Bipolaris maydis. This was a minor disease of corn for many years, with no economic effect on yield. In 1970, a highly virulent strain called Race T appeared on corn hybrids with Texas male-sterile cytoplasm. Losses were severe.
Symptoms of Race lesions are spindle-shaped with yellow-green halos. They will later have dark, reddish-brown borders and occur on leaves, stalks, leaf sheaths, husks, and shanks. Cob Rot ear occurs as well as ear drop. The fungus overwinters in corn debris and on the seed. Wind and splashing water spread the spores rapidly in the field under ideal conditions, cycling in about 72 hours. Control with resistant hybrids.
Southern corn leaf blight (SCLB) is favoured by warm temperatures (68-90°F or 20-32°C) and high humidities. Thus, it tends to be a problem in the Southern half of Illinois, although it can be found farther north if weather conditions are favourable. Frequent rainy periods enhance disease development.
The symptoms of SCLB are leaf lesions ranging from minute specks to spots of one-half inch wide and one and one-half inches in length. They are oblong, parallel-sided, and tan to greyish in colour. A purplish to brown border may appear on the lesions depending on the genetic background of the plant. Early and severe infections in susceptible plants predisposes them to stalk rots.
The fungus overwinters in corn debris as spores or mycelium. Spores are spread by wind or splashing water to growing plants. After infection and colonization, sporulation from these primary lesions serves as the source for secondary spread and infections as long as weather conditions are favourable for disease development and living tissues are present. The disease cycle may repeat every few days under ideal conditions. Germination of spores and penetration into the plant can occur within six hours when free water is present on the leaf surface and temperatures are between 15 and 28°C. Control of SCLB is easily accomplished with resistant hybrids. Although some slight flecking may be found in some hybrids, this is simply a part of the resistant reaction and does not lead to any economic loss.
Burial of crop residue is helpful where erosion is not a problem. Crop rotation is especially suggested where no-till is used or where heavy crop residues are found. Since this fungus overwinters on debris, the planting of corn into such residues may result in earlier infection and poor seedling performance. Foliar fungicides are useful in seed production fields. For optimal control, it is important to control foliar disease during the period from 14 days before tasselling to 21 days after tasselling. Research has shown that this four-week period is the most critical for leaf blight damage and that yields and quality are most affected if susceptible inbred are not protected at this time.
This disease needs free water (leaf wetness) to infect plant tissue. Therefore conditions for starting an infection are: leaf wetness and temperatures warmer than 15°C. The infection will be finished after we can accumulate 100 degreehours (temperature and leaf wetness hours).
START SCLBinfection: if leaf wetness > 0 and air temperature >= 15 then begin
INCREASE of INFECTION: SCLBinfection actual hour =SCLBinfection former hour + (air temperature * leaf wetness period in minutes)
STOP: if conditions are not given (no leaf wetness, temperature below 15°C), or if SCLBinfection > 100*60 °C (= degreeminutes) then SCLBinfection = 100*60 (in sum 6000 degree minutes or 100 degreehours (wich is for example about 6hours leaf wetness at 15°C or 5hours leaf wetness at 18°C), of leaf wetness at a temperature higher than 15°C is the maximum).
(If Temperature is higher than 15°C and leaf wetness for more than 100*60°C accumulated Temperature per minute infection is complete)
These models have been designed following the Report on Plant Diseases 202 – Common Leaf Blights and Spots of Corn by Mohammad Babadoost.
Extension Specialist in Fruit and Vegetable Diseases, Department of Crop Sciences, the University of Illinois at Urbana-Champaign.
Northern corn leaf blight (NCLB), caused by the fungus Exserohilum turcicum previously called Helmithosporium turcicum, can cause yield losses in humid areas where corn is grown. In moderate humid climate the disease can occur everywhere, but usually does not appear in fields before silking. This disease rarely causes significant yield losses during dry weather, but during wet weather periods it may result in losses of over 30%, especially when established on the upper leaves at the silking stage of maize development. If leaf damage is only moderate or is delayed to about 6 weeks after silking, yield losses are mostly minimal. Northern corn leaf blight also predisposes corn to stalk rot by increasing stress on the plants.
Exserohilum turcicum is able to infect the leaves of the corn plant when temperature and leaf wetness conditions are given. The time span to infect plant tissue is depending on the actual air temperature and leaf wetness period. The infection can take place, if temperature is ranging from 13°C to 28°C. At a temperature of 18°C it will need 18 hours for infection. If the temperature is lower it will need even longer time and if it is warmer it will need a shorter leaf wetness period.
START of the calculation of a NCLBinfection: If (leaf wetness > 0) and (air temperature >= 13) and (air temperature <= 28)INCREASE: NCLBinfection actual:=NCLBinfection of the former hour +((air temperature-13)*leaf wetness duration in minutes).STOP: no leaf wetness, temperature not in the optimal range or if NCLBinfection > (18-13)*18*60 then NCLBinfection = (18-13)*18*60. That means that in sum 5400 degreeminutes (= 90degreehours) could be reached then an infection is fulfilled (= (18°C- 13°C)* 18hours leaf wetness in minutes (18*60). At 18°C it needs 18hours of leaf wetness for an infection. If the temperature is higher or leaf wetness periods are longer it is still 5400 (all values above are 100% infection).
These models have been designed following the Report on Plant Diseases 202 – Common Leaf Blights and Spots of Corn by Mohammad Babadoost.
M. Babadoost is an Extension Specialist in Fruit and Vegetable Diseases, Department of Crop Sciences, the University of Illinois at Urbana-Champaign
FUSARIUM SSP. ON CORN
Fusarium Ear Rot is the most common fungal disease on corn ears. It is caused by several species of Fusarium. Symptoms of Fusarium ear rots are a white to pink- or salmon-coloured mold, beginning anywhere on the ear or scattered throughout. Often the decay begins with insect-damaged kernels. Usually, it does not involve the whole ear. Infected kernels are often tan or brown, or have white streaks. These fungi can produce mycotoxins.
The most common ear diseases in the Midwest are caused by Fusarium spp. fungi. “Fusarium ear rot” or “Fusarium kernel rot” refer to the disease caused by Fusarium verticillioides, F. proliferatum and F. subglutinans. Symptoms of Fusarium ear rots are a white to pink or salmon-colored mold, beginning anywhere on the ear or scattered throughout. Often the decay begins with insect-damaged kernels, but most of the ear can be affected. Infected kernels are often tan or brown-coloured, or have white streaks. Fusarium species also cause corn stalk rots. The fungi survive on corn residue and the residue of other plants, especially grasses. Fusarium spores are spread by wind and splashing rain to the silks, which are most susceptible as they are senescent and turn brown. Infection of kernels can also occur through the stalk, but this mode of kernel infection is less common. Insect damaged kernels are very susceptible, and spores carried on the insects can infect the damaged kernels.
Gibberella ear rot of corn is caused by the fungus Gibberella zeae, also known as Fusarium graminearum.
Maisfusarium1 1It can be identified most readily by the red or pink color of the mold. It almost always begins at the tip of the ear. In some cases, the color is too pale to bee seen readily, so the mold appears white. In this case, it may not always be possible to distinguish Gibberella ear rot from Fusarium ear rot without microscopic examination.
This fungus also causes corn stalk rot and Head Scab of wheat; it survives in corn and small grain residue.
The spores of this fungus infect through silks; stalk infections are not believed to lead to Gibberella ear rot. Spores reach the silks by splashing water or they may be carried by insects. Infections may originate at insect wounds rather than at the ear tip.
Gibberella ear rot infections occur more commonly when the weather is cool and wet during the first five days after silking. Continued development of the mold also depends on subsequent cool, wet weather. Optimum temperatures for disease development are 19°C – 24°C.
Corn and Wheat is susceptible for fusarium infections in BBCH stage 61 to 75. After stage 75 the model is only used in case of heavy infestations of the European Corn Borer.
The fungal agents belonging to the Fusarium Head Blight complex on corn are known to be favoured by warm temperatures of 20 °C to 30 °C and long humid periods. Several day long leaf wetness periods will lead to early visible symptoms. But Symptoms can be seen after a longer latency period if the infection is followed by and leaf wetness period of 18 hours or even shorter. Symptoms can be found after artificial inoculation at temperatures of 15°C too. Summarizing all different temperatures and moisture combination, which we found in literature we decided to point out Fusarium Head blight Infections if temperature and leaf wetness period or periods with more than 85% relative humidity are exceeding the values visualised in the following graph.
Infections are started when the air temperature is between 2 and 32°C and rain sum by 2 mm or more or rel. humidity of more than 85%. Infection increases when rel. humidity stays equal or more than 85% (when no leaf wetness occurs). But if rel. humidity decreases lower as 85%r.h. the infection progress stops.
Fusarium Head Blight Infection can be assumed if the infection progress value reaches 100%. The calculation of this progress value follows the relation in between the duration of moist conditions and the temperature displayed in the graph above.
This model is used to visualise the infection days and the climate conditions. The grower’s knowledge about the development stage of the different wheat varieties he grows, which gives him the possibility to access if he should apply a curative spray immediately after infection.
Infektion und Ausbreitung von Fusarium spp. an Weizen in Abhängigkeit der Anbaubedingungen im Rheinland, Lienemann, K.; E.-C. Oerke und H.-W. Dehne
Water activity, temperature, and pH effects on growth of Fusarium moniliforme and Fusarium proliferatum isolates from maize, Sonia Marin, Vicente Sanchis, and Naresh Magan
Interaction of Fusarium graminearum and F. moniliforme in Maize Ears: Disease Progress, Fungal Biomass, and Mycotoxin Accumulation, L. M. Reid, R. W. Nicol, T. Ouellet, M. Savard, J. D. Miller, J. C. Young, D. W. Stewart, and A. W. Schaafsma
Evaluierung von Einflussfaktoren auf den Fusarium-Ährenbefall des Weizens, Wolf, P. F. J.; Schempp, H.; Verreet, J.-A.
Infection trials with extended leaf wetness periods of Fusarium Head Blight lead to high contents in mycotoxin. Therefore a leaf wetness period of 48 hours or more during stage 61 and 69 is assumed to give a high risk of mycotoxins. Analyses of the DON content in commercially grown wheat showed, that leaf wetness periods long enough for infection, following an initial infection in stage 61 to 69 can increase the DON values too. In the case of longer leaf wetness periods, mycotoxins can increase up to stage 75. Later infections will only be possible in interaction with European Corn Borer.
Fieldclimate accumulates a risk figure proportional to the infection progress for every successful infection period over the period, which has been selected to be appropriate for these calculations. A risk of 100% is given, when six completed infection periods have been fullfiled. Normally the leaf wetness period leading to a Fusarium infection is longer than in minimum needed. Therefore most Fusarium infections will lead to an risk increase of more than 17%. The risk value pointing to a problematic mycotoxin situation is depending on the field history. Corn grown after no-tillage corn can only carry a small risk if it is not sprayed at the optimum situation. In not sprayed corn we have to expect increased DON values already after 35% of the risk. Tillage Corn after tillage corn can carry a higher risk of 70%. First-year corn should be tested for DON if the risk exceeds 100%.
Fusarium bei Mais – ein ernstzunehmendes Problem, Kolbenfusariose, Fusarium, Mais, DON, Mykotoxine, Fumonisine, Höchstwerte, Landwirtschaftliches Technologiezentrum Augustenberg (LTZ), Neßlerstr. 23-31, 76227 Karlsruhe
GREY LEAF SPOT
Gray leaf spot of corn is caused by the fungal pathogen Cercospora zea maydis.
The disease development depends on the variety of the maize hybrid and the weather conditions. That is the reason for a local severity infection in some places, wherein others no infection was taken place.
Symptoms are seen on the leaves as small, point lesions surrounded by yellow halos. It is sometimes difficult to identify the disease at that stage. When lesions mature, they elongate into narrow, rectangular, brown to gray spots and they expand parallel to leaf veins and become about 3,5 – 5cm long. Symptoms depend on the hybrid susceptibility and vary among them. Symptoms of gray leaf spots could be similar to other foliar fungal diseases such as anthracnose leaf blight, eyespot, or common rust. Symptoms of Gray leaf spot on maize Infection cycle of the fungal pathogen – The fungus survives during the winter on infected corn residues on the soil surface. With increasing temperatures in the spring spores are produced on maize residues and they splash to young leaves. That’s the reason why symptoms are commonly observed on the lower leaves of the plant. Spores are transported by wind or rain. – Infections occur during long warm and humid conditions.
The temperature of 24°C to 30°C is optimal for the fungus as well as the relative humidity of more than 90%. – Symptoms are seen after long periods of heavy dew, in areas where humidity is very high and dew persists long in the morning. – C. zeae-maydis spores decrease development during periods with low humidity, but the infection process starts again under high relative humidity. Each lesion can produce many spores, which are splashed by rain or transported by wind to young healthy leaves. There they survive until conditions are favorable for infection. – Due to the length of the infection process, symptoms may not be seen for some weeks after infection, depending on weather conditions and hybrid susceptibility. – Hot, dry weather will restrict disease development and distribution to other plants. – Gray leaf spot can have a great impact on the yield. The correlation of infected leaf tissue and the amount of yield is not clear. The lesions reduce the capacity of the plant of photosynthetic areas on leaves. Reduced photosynthesis could also lead to stalking rots and lodging. Conditions in Fieldclimate: – Hours between 22°C and 30°C/2,305*2 plus hours of rel. humidity (more or equal of 90%) during night/ 3,30*2 – range from 0 to 100%
Gray Leaf Spot of maize Cercospora zeae-maydis Risk Model:
- Calculation performed in the time between stage V4 and V12
- Number of hours with temperatures from 22°C to 30°C divided by (2.305*2) plus hours of relative humidity > 90% during night divided by (3.30*2). This value can
- range from 0 to 100.