Symptom Identification and Control Methods for Impatiens Downy Mildew

[Symptoms]

Mainly affects the stem. At first, oily dark green streaky patches appear at the root-stem junction, rapidly expanding into irregular dark brown areas; in severe cases, the entire stem and leaves turn black, the plant quickly collapses, and the root crown becomes sunken, causing the plant to lodge and die. Under saturated humidity persisting for a long time, sparse white mold grows on the diseased areas, which are the pathogen's sporangiophores and sporangia.

[Morphological characteristics of the pathogen]

It grows rapidly and produces abundant spores on CMA, OMA, and V4A media, while on PDA medium it grows slowly and does not produce oospores. Colonies are white and cottony; hyphae are colorless, non-septate, and branched; sporangiophores are colorless, non-septate, unbranched, 4-5 um wide, each bearing one sporangium; sporangia are ovoid to elliptical, terminal, with a short pedicel, shed after maturity, with a distinct papilla-like protrusion at the apex, measuring 32-54 x 19-30 (um), and after germination produce more than 50 zoospores; some sporangia can also germinate directly to produce 1. sporangium 2-3. sporangium and zoospores 4-6. latent germ tube penetrating; oogonia are spherical, colorless to pale yellow, thin-walled, measuring 28 oogonium, lateral antheridium and oospore 7. oogonium and surrounding antheridium -32 um. The pathogen's growth temperature range is 10-32 degC; hyphal growth is fastest at 25 degC. It adapts to pH 3.5-11.5, with the optimum at pH 6; hyphae cannot grow at pH 3 or pH 12. The fungus is killed at 50 degC for 20 minutes or 45 degC for 40 minutes.

[Transmission routes and disease conditions]

The pathogen overwinters on seeds or as mycelium and oospores with diseased plant residues in the soil. The fungus's mycelium and sporangia cannot overwinter in soil, but oospores can survive in soil for 4 years. Oospores are the main source of primary infection for this fungus, spread by wind, rain, and irrigation water. Zoospores can directly invade the roots of impatiens and are the main organs of re-infection. Sporangia germinate and release 10-36 zoospores; sporangia can also germinate directly to produce germ tubes and appressoria, producing penetration pegs that enter leaf tissue through leaf stomata, functioning like conidia. Completion of the above process requires the presence of water droplets. Various wounds on stems and leaves can worsen the disease. The pathogen infecting seedlings causes lodging; at the adult plant stage, disease is severe when there is abundant rain, high humidity, or heavy clay soil, low-lying terrain, or continuous cropping.

[Epidemic dynamics]

At an air temperature of 20-25 degC and continuous moisture for 15 hours, 50% become infected. Temperature and the duration of humidity and their interaction greatly affect the infection efficiency of this fungus; the longer the humidity persists, the more favorable it is for pathogen infection. The optimum temperature for pathogen development is 27 degC-31 degC, with a maximum of 36 degC and a minimum of 10 degC. When the air temperature is below 20 degC or above 25 degC, a longer dew duration is needed for rapid infection. Disease is likely to occur after the rainy season begins or after heavy rainstorms.

[Control methods]

1. Remove diseased leaves; apply more phosphorus and potassium fertilizers, etc. Reduce the sources of primary infection. Maintain good garden sanitation, use soil that has been left fallow for a year, and plow the land multiple times in midsummer; after sufficient sun exposure, the fungus's viability decreases, and sunlight exposure can cause dormant oospores to germinate, thereby losing their infective ability; thoroughly clear diseased plant residues in autumn to reduce disease occurrence.

2. At the early stage of disease, spray 75% chlorothalonil wettable powder at 500-600 times dilution, or 40% triethyl phosphonate aluminum (phosethyl-Al) wettable powder at 200 times dilution, 70% phosethyl-Al mancozeb wettable powder at 500 times dilution, 58% metalaxyl mancozeb at 400-500 times dilution, 72.2% propamocarb aqueous solution at 700-800 times dilution, 64% oxadixyl mancozeb wettable powder at 500 times dilution, or 72% cymoxanil or cyazofamid or cymoxanil mancozeb wettable powder at 600 times dilution, 60% flumorph mancozeb wettable powder at 800 times dilution, 69% dimethomorph mancozeb wettable powder at 800 times dilution.

3. Preventing rain splash and spread plays an important role in reducing aboveground disease occurrence.