Wheat Disease Update

Published: May 29, 2015

The majority of the winter wheat in Missouri is well past flowering so it is too late to be considering fungicide applications.  But the recent spell of alternating warm drier weather with cool, wet weather has led to the development or increase in some wheat diseases.  It is still important to check wheat fields and note which varieties are showing high levels of stripe rust, Fusarium head blight or other diseases and which are showing better resistance to these diseases for future planning.

The foliage disease which seems to be of most concern is stripe rust.  There are been scattered reports of Septoria leaf blotch and leaf rust.  Although we haven’t received many reports or samples of diseased wheat heads yet, the weather has certainly been favorable for the development of Fusarium head blight, Stagonospora glume blotch and bacterial black chaff.

Stripe rust, caused by the fungus Puccinia striiformis, has become more prevalent in Missouri over the last few years.  Stripe rust may develop earlier in the season than leaf rust or stem rust.  The pustules of stripe rust are yellow or yellowish-red and occur in obvious stripes or streaks running lengthwise on the wheat leaves.  This disease is more commonly associated with cooler temperatures, especially cooler night temperatures. The stripe rust pathogen does not survive in infested residue left in a field.  Rather, stripe rust is reintroduced into this area each season when spores are carried up on air currents from the southern United States. Following reports of stripe rust development in southern states or southern portions of Missouri may be helpful in making the decision to apply a foliar fungicide for stripe rust management.

Fusarium head blight or Scab:  Fusarium head blight or scab of wheat develops on plants in the flowering to early grain fill stages of growth.  Infection is very dependent on environmental conditions while wheat is in susceptible stages of growth, i.e. flowering.  Moderate temperatures in the range of 77-86°F, frequent rain, overcast days, high humidity and prolonged dews favor infection and development of scab.   Weather conditions over the next several weeks will determine the extent and severity of scab in this year’s wheat crop.  Fusarium head blight or scab problems will be more severe if rains coincide with flowering of wheat fields. If the rain continues as the crop moves through the flowering stages, the risk for scab will increase.

The characteristic symptom of scab on wheat is a premature bleaching of a portion of the head or the entire head.  Superficial mold growth, usually pink or orange in color, may be evident at the base of the diseased spikelets.  Bleached spikelets are usually sterile or contain shriveled and or discolored seed.  The susceptibility or resistance of the variety to Fusarium head blight will have an effect on how much of the head develops symptoms.  On susceptible varieties, initial infection may be confined to several spikelets but if weather conditions remain favorable, the entire head may eventually be infected.  Varieties with more resistance may only have individual spikelets showing symptoms.

Scab is caused by the fungus Fusarium graminearum.  This fungus overwinters on host residues such as wheat stubble, corn stalks and grass residues.  Spores are carried by wind currents from the residues on which they have survived to wheat heads.  If environmental conditions are favorable, i.e. warm and moist, the spores germinate and invade flower parts, glumes and other portions of the spike.  Scab infection occurs when favorable environmental conditions occur as the wheat crop is in the flowering to early grain fill stages.

Unfortunately, the detrimental effects of scab are not limited to its adverse effects on yield.  The fungi which cause scab may also produce mycotoxins.  Vomitoxin (deoxynivalenol or DON) and zearalenone may occur in wheat grain infected by scab fungi. This is a primary concern where grain is fed to non-ruminant animals.  Ruminants are fairly tolerant of these two mycotoxins.  Also, the fungi which cause scab may survive on the seed and can cause seedling blight and root rot problems when scabby grain is used for seed.

Crop rotation, variety selection and residue management are preventative measures for managing scab in wheat.  At this point in the season the only remaining management option would the application of a fungicide to try to reduce scab levels.  The fungicide table in this issue of the Integrated Pest & Crop Management Newsletter lists the fungicides labeled for the suppression of Fusarium head blight or scab.  Growers should be scouting fields to get a feel for incidence and severity of scab in this year’s wheat crop. Because of possible mycotoxin concerns and seed quality concerns, grain from fields with scab may require special handling.  Wheat planted on corn, sorghum or wheat residue (even wheat double cropped with soybeans) has a greater risk for scab.

Other Head Diseases of Winter Wheat:

From flowering through the early stages of grain fill is also the time to scout for other head diseases of wheat such as loose smut, Septoria and Stagnospora infections on heads, bacterial stripe and black chaff on heads and take-all.

Loose smut is obvious as heads emerge from the boot and for several weeks after that.  The kernels on infected heads are replaced with masses of powdery black spores.  So the heads have a very distinct black, powdery appearance. These spores are eventually dislodged by wind and rain, so later in the season the smutted stems are less evident and only the bare rachis will be left. Spores produced on smutted heads are wind carried to adjacent plants in the field and infect through the flowers.  The fungus that causes loose smut survives within the embryo of wheat seeds.  If infected seed is planted, the plants growing from those seeds will be infected and develop smutted heads the next season.  If seed from a field that has a “small” amount of smut in one season is used for seed, the field planted with that seed may have a substantially higher level of smut.  Loose smut is best controlled by planting either disease-free seed or using a systemic fungicide seed treatment.

Glume blotch:  Septoria leaf blotch is present in the lower canopy of many fields this year.  It hasn’t seemed to move up in the canopy to the flag leaf or head but with increased precipitation and high humidity it could still develop on flag leaves and heads.  On the heads dark brown to black blotches may develop.  Stagnospora nodorum may also cause leaf lesions but is usually more common on heads- again causing dark blotches on glumes of part or all of the head.

Bacterial stripe or black chaff is a bacterial disease that produces symptoms on both leaves and heads.  Water-soaked lesions may develop on young leaves. These expand into reddish-brown to brownish-black streaks on the leaves.  Glumes and awns show brown-black blotches or streaks.  Fungicides are not effective against bacterial stripe or black chaff so the use of resistant or tolerant varieties and crop rotation are the main management options.