Figure 19. Lodging of corn plants due to stalk rot damage. Photo courtesy of MAFRI. Figure 20. Shredding of internal tissues of a corn stalk by a stalk rot pathogen. Photo courtesy of MAFRI. STALK ROTS When to look for: Mid-August to Early October While the stalks contain a high level of carbohydrates, corn plants are able to tolerate most stalk rot organisms. As grain filling reduces the reserves in the stalks, susceptibility to infection increases. Stalk rots are generally observed later in the season; often some time after the infection has taken place, when the grain is drying down in the field. Scouting for Corn Stalk Rots When scouting the crop, notice general wilting of the plant. This will occur 40 60 days after pollination. Over the next few days, leaves will appear grey, the ear drops, and the outside of the lower stalk turns brown. When the outer stalk tissue has turned brown, the pith tissue in the lowest internode is rotted and separated from the rind, eventually hollowing, therefore weakening the stem. Two methods can be used for scouting for stalk rots: The Push Test Randomly select 20 plants from 5 areas of the field. Push the top portion of the plant and note whether the plant lodged or not. Record the number of lodged plants. The Pinch Test Randomly select 20 plants from 5 areas of the field. Remove lower leaves and pinch or squeeze the stalk above the brace roots. Easily squeezed stalks are rotting on the inside. Record the number of rotted stalks. If 10-15% of the plants are affected, then early harvest should be considered to avoid difficulty with lodged plants. Also, the extra drying costs that may be incurred will be covered by the increased efficiency of harvest. 33
Anthracnose Stalk Rot (Colletotrichum graminicola) Symptoms Several internodes of the plant may be infected. Lodging before maturity. Sometimes portions of the plant above the ear die while the lower portions remain green. Shiny black discolouration at the base of the stalk initially and later in the top killed portions of the plant. Blackened areas may be in uniform or in patches. Internal stalk tissues are often blackened or discoloured, appearing shredded. Stalk is easily squeezed between thumb and forefinger. Figure 21. Midseason symptoms of anthracnose stalk rot, called top dieback. Photo courtesy of Pioneer Hi-Bred. Figure 22. Anthracnose stalk rot on mature corn stalk. Photo courtesy of Pioneer Hi-Bred. Figure 23. Anthracnose stalk rot disease cycle. Photo courtesy of Pioneer Hi-Bred. The fungus can survive in some weed species, infected seed and corn residues. It can also cause leaf blight. Spores produced in leaf lesions can splash behind the leaf sheaths where the stalks are then penetrated. Insect feeding sites and other wounds also serve as entry points. Infested corn residues generally create infection in new crops in the nearby area. The fungus can also enter new fields by wind dissemination of spores associated with dry leaf pieces or by penetration of the roots by buried inoculum. Management Crop rotation Minimize crop stress balanced fertility, European corn borer control (insect damage), etc. 34
Fusarium Stalk Rot (Fusarium moniliforme, F. subglutinans, F. profileratum) Symptoms Shredding of internal tissues Brown streaks on lower internodes Rotting stem tissues may appear a variety of colours, from beige to whitish pink to orange pink, which tend to be less intense that the pink-red stain of Gibberella stalk rot. Decay begins after pollination, increasing in severity as the plants mature, becoming more prevalent under warm, dry conditions. Figure 24. Fusarium stalk rot on corn stalk. Photo courtesy of Pioneer Hi-Bred. Fungi survive in soil and on crop residue. Favourable conditions encourage infection of roots or wounds on stalks or leaves. Spore dispersal can occur by wind, rain, insects or birds. Figure 25. Fusarium stalk rot disease cycle. Photo courtesy of Pioneer Hi-Bred. Management Stress-reducing management practices, avoiding injury to the plant, especially the roots. Use of hybrids resistant to other stalk rot diseases usually display resistance to Fusarium stalk rot. 35
Diplodia/Stenocarpella Stalk Rot (Stenocarpella maydis) Symptoms Initially, a brown to tan discolouration of lower stem, internal tissues of the lower plant portions appear shredded and easily crushed. White strands or mats of fungal growth, especially during extended moist periods. Tiny dark brown to black spots will appear on lower portions of the plant, like Gibberella. Figure 26. Corn stalk showing symptoms of diplodia stalk rot. Photo courtesy of Pioneer Hi-Bred. Figure 27. Corn stalk with diplodia stalk rot symptoms. Photo courtesy of Pioneer Hi-Bred. Fungi survive on stalk debris either in or on the soil surface. In warm moist conditions and rain, the spores can be released and spread by wind or insects. Wet weather 2-3 weeks after silking is suitable for development of Diplodia stalk rot. Infection is usually via the crown, roots or lower nodes. Insect damage, or any type of open wounds, also provide entry points for the fungus. Management Figure 28. Diplodia stalk rot disease cycle. Photo courtesy of Pioneer Hi-Bred. Reduction of any stress on the plant is the best method of management. Varieties that have resistance to Gibberella stalk rot will usually display comparable levels of resistance to Diplodia stalk rot. Rotations of at least a year, or reduction of debris in the field may reduce outbreaks of the disease. 36
Gibberella Stalk Rot (Gibberella zeae) Symptoms Wilting leaves, resembling frost damage and appearing light to dull grey-green in colour. Lower portion of stalk softens and turns a light brown colour Nodes and surrounding areas, tiny superficial round black specks (perithecia) are produced, easily scraped off of stalk surface. Internal pith tissue breaks down, leaving only the thread-like vascular bundles. Reddish-pink discolouration observed inside the stalk is a classic Gibberella symptom Figure 29. Giberella stalk rot on interior and exterior of corn stalk. Photo courtesy of Pioneer Hi-Bred. Figure 30. Cross section of giberella stalk rot on corn stalk. Photo courtesy of Pioneer Hi-Bred. Fungi survive on infested overwintering crop residues. Inoculum may be produced as splash dispersed conidia in the summer. Stalk infections usually occur shortly after pollination, developing at the base of the leaf sheaths or near the brace roots. Fungus may also enter through the roots and grow up into the lower stem. G. zeae also causes scab and seedling blights of wheat, barley, oats and rye. Figure 31. Giberella stalk rot disease cycle. Photo courtesy of Pioneer Hi-Bred. 37
Management The fungus that causes Gibberella stalk rot is the same fungus that causes Fusarium head blight in cereals. Crop rotations of a minimum of one year between cereal or grass crops and corn are necessary in prevention, especially if reduced tillage is practiced. Plowing under of residue and controlling weeds and volunteers will reduce inoculum levels. Reduce crop stress. Figure 32. Giberella stalk rot. Photo courtesy of Pioneer Hi-Bred. Figure 33. Fusarium stalk rot. Photo courtesy of Pioneer Hi-Bred. 38
Pythium Stalk Rot (Pythium aphanidermatum) Symptoms generally restricted to the internode closest to the soil line Soft, collapsed and dark green tissues. Even after lodging, the plants usually remain green as the vascular system remains intact. No apparent signs of overwintering structures of the fungus. If infection occurs before flowering, the lowest internode decays, and infected stalks may have a strong odour. Internodes may twist, causing plant to lodge, but greenishbrown stem colour remains for weeks. Should infection occur during milk stage (R3), the roots and several lower internodes will become water soaked, leading to early plant death. Figure 34. Pythium stalk rot on corn plant. Photo courtesy of Iowa State University Extention and Outreach. Species of Pythium are considered water moulds, acting differently from other stalk rot fungi. Spores that can move on their own (zoospores), swim through moist soil, towards the roots of a corn plant, gain entry inside the plant and initiate disease. Fungi overwinter as oospores, tiny hardened structures, which can survive dry cold conditions of winter in soil and crop residues. In spring, oospores germinate, releasing the zoospores or threads of mycelium, each capable of infecting plants. Oospores may remain viable for years, and the fungus may be able to survive on weeds. Management Avoid poorly drained areas or improving soil drainage could prove useful. 39
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