5.4 Plant Diseases, Nematodes & Vertebrate Pests
Key Takeaways
- The disease triangle requires a susceptible host, a virulent pathogen, and a favorable environment; removing any one prevents disease.
- Protectant fungicides must be present before infection, while systemic fungicides offer limited curative activity, so disease control is fundamentally preventive.
- Abiotic disorders such as compaction, drought, road salt, and herbicide injury mimic disease but cross species lines and lack signs of a pathogen.
- Nematode damage appears as patchy stunting, chlorosis, and poor response to fertilizer and water, and is confirmed by a soil assay rather than by symptoms.
- Ohio Category 7 Vertebrate Animal Control covers pest birds, rodents, and other vertebrates including moles and Canada geese, and excludes fumigants.
5.4 Plant Diseases, Nematodes & Vertebrate Pests
Three pest groups that reward a different diagnostic reflex than insects and weeds.
Plant Pathogens and the Disease Triangle
A plant disease requires three conditions simultaneously:
- A susceptible host
- A virulent pathogen
- A favorable environment - usually a specific combination of leaf wetness duration, temperature, and humidity
Remove any leg and disease does not develop. That is why cultural changes - improving air movement, watering early in the day so foliage dries, avoiding overhead irrigation at dusk, spacing plants, choosing resistant cultivars - are legitimate disease control tactics and not just good advice.
The pathogen groups
| Group | Signs to look for | Control implications |
|---|---|---|
| Fungi | Mycelium, spore masses, mildew, rust pustules, lesions with defined margins, fruiting bodies | Most fungicides target fungi; protectant products must be on the surface before spores land |
| Bacteria | Water-soaked lesions, ooze or slime, angular spots bounded by veins, foul odor in soft rots | Few effective curative products; sanitation, pruning in dry weather, resistant cultivars |
| Viruses / phytoplasmas | Mosaic, mottling, ring spots, distorted or strapped growth, witches' broom; no signs of an organism | No chemical cure. Manage the insect vector and remove infected plants |
| Nematodes | Root galls, lesions, stubby or proliferated roots | Sampling-driven; often pre-plant management |
Protectant versus systemic fungicides
- Protectant (contact) fungicides sit on the plant surface and prevent spore germination and penetration. They must be applied before infection and must be reapplied as new tissue expands and as rain removes residue.
- Systemic fungicides are absorbed and redistribute in the plant, offering limited curative or "kick-back" activity shortly after infection - but "limited" is the key word.
Because most disease control is preventive, a disease call is often a call the applicator will not be able to fix this season. Say so honestly rather than treating for the sake of treating - ORC 921.24(F) prohibits applying known ineffective materials.
Abiotic Disorders: The Great Imitator
Roughly a large share of plant problems in Ohio landscapes are not caused by any pest at all.
| Abiotic cause | Tell-tale pattern |
|---|---|
| Drought or waterlogging | Follows soil texture, slope, and irrigation coverage |
| Soil compaction | Confined to traffic lanes, staging areas, or new construction footprints |
| Road or sidewalk deicing salt | Damage on the road-facing side, worst at the plow-spray line |
| Girdling roots or planting too deep | Slow decline over years; no trunk flare at grade |
| Herbicide injury | Crosses unrelated species, follows a gradient from the source or a sprayer pattern |
| Nutrient deficiency | Uniform, follows leaf age - older or newer leaves depending on nutrient mobility |
The discriminator is the pattern. Infectious disease usually spreads within a susceptible species over time. Abiotic damage appears at once, on multiple unrelated species, and matches a physical feature of the site.
Nematodes
Plant-parasitic nematodes are microscopic roundworms feeding on roots. Above ground they produce nonspecific symptoms - patchy stunting, chlorosis, thin turf, wilting during heat, and a poor response to fertilizer and irrigation. Below ground look for galls (root-knot), lesions, stubby roots, or excessive branching.
Symptoms alone never confirm nematodes. A soil and root assay through a diagnostic lab is required, and sampling technique matters: sample the edge of a declining area, not the dead center, take cores from the root zone, keep samples cool, and submit promptly.
Vertebrate Pests and Ohio Category 7
OAC 901:5-11-01(P)(7) defines Vertebrate Animal Control as the application of pesticides other than fumigants for the control of pest birds - inside or outside structures - and for the control of rodents and any other vertebrates. ODA's category description adds explicitly: moles and Canada geese included.
| Vertebrate pest | Signs | Notes for the Ohio applicator |
|---|---|---|
| House mouse | Droppings 3 to 6 mm, gnaw marks, rub marks, nesting material | Curious and explores new objects; bait and trap placement every 6 to 10 feet |
| Norway rat | Droppings 18 to 20 mm with blunt ends, burrows near foundations, greasy rub marks | Neophobic - avoids new objects for days; pre-baiting improves acceptance |
| Roof rat | Pointed-end droppings, aerial runways | Less common in Ohio than Norway rat |
| Moles | Surface tunnels and volcano-shaped mounds | Insectivores, not rodents; they are after grubs and earthworms, so grub control alone rarely removes them |
| Voles | Surface runways in turf under snow, girdled bark at the base of trees | Damage is often discovered at snowmelt |
| Pest birds (pigeon, starling, house sparrow) | Droppings, nests in signage and ledges | Exclusion, netting, and ledge modification first; many native birds are federally protected |
| Canada geese | Turf grazing, droppings, aggression during nesting | Federally protected migratory birds - coordinate with the Ohio Division of Wildlife before any control |
Two cautions the exam likes:
- Rodenticide placement is legally constrained. In Ohio schools, only paraffin-based rodent products in industry-identified tamper-resistant bait stations, or rodenticides placed in wall voids and other areas inaccessible to humans and domestic animals, may be used while school is in session.
- Legal protection cuts across pest status. Most native birds are protected under the federal Migratory Bird Treaty Act, and state wildlife law governs many mammals. Being licensed in Category 7 does not by itself authorize taking a protected species.
Where Fumigants Fit
Category 7 excludes fumigants. Burrow fumigation and structural fumigation require the fumigation categories - 2F Soil Fumigation, 10C Fumigation, or 6D Greenhouse Pest Control - under OAC 901:5-11-02.
A row of arborvitae shows browning on the side facing a county road, worst at the height of plow spray. Several unrelated shrubs nearby show the same pattern. What is the most likely cause?
Which control approach is appropriate for a landscape plant confirmed to be infected with a plant virus?
An Ohio homeowner reports surface tunnels and volcano-shaped mounds in the lawn. Which statement is correct?