Nursery Pest and Disease Update 5/15/2023

DISEASES ON THE RADAR

Leaf spot / foliage disease management

• About: Most leaf spots are caused by several different genera of fungi, including but not limited to Alternaria, Colletotrichum, Cercospora, Diplocarpon, Phoma, Phyllosticta, and Septoria. There are also bacteria that can cause leaf spot (Psuedomonas and Xanthamonas) and even foliar nematodes (Aphelenchoides). Some common ornamental species that are regularly affected by leaf spot / foliage pathogens include maple, hydrangea, cherry, crape myrtle, redbud, viburnum, oak, Virginia sweetspire, and rose, though there are many other plants that can also be infected.
• Cultural management: While the symptoms may not appear until later in the season, the infection period starts in the spring. Reducing the leaf wetness period by limiting overhead irrigation and increasing airflow can help to prevent these diseases, as well as proper sanitation by removing any diseased leaves from the area.
• Material considerations: Protective fungicides and/or bactericides can also be effective if applied early in the season before the symptoms develop. Materials include copper hydroxide, mancozeb, propiconazole, chlorothalonil, myclobutanil, azoxystrobin, or thiophanate-methyl.

Fire blight on Rosaceae – (modified from Dr. Ann Gould’s original post – click here)

• About: Multiple reports of fireblight have been noted due to the ideal conditions in NJ as of late, warm (65 75F) and wet conditions. This bacterial pathogen (Erwinia amylovora) only affects plants in the rose family (Rosaceae), apple, crabapple, cotoneaster, hawthorn, mountain ash, pyracantha, and pear as examples. Twigs and branches infected with the pathogen die rapidly and appear scorched, hence the name “fireblight”, additionally tender branches droop as they are infected leading to the shepherd’s crook.  As the disease advances, cankers form at the base of infected branches, and highly susceptible plants may die. Sticky bacterial ooze can be spread to all other plant parts by insects (highly attracted to bacterial ooze), wind, splashing rain, or pruning equipment.
• Cultural management: To manage the disease, improve plant vigor, but avoid heavy spring fertilization that promotes succulent growth. In late summer (after bacterial oozing no longer occurs), prune all diseased wood at least 6 to 8 inches below the infection, surface sterilizing tools between cuts (EXTREMELY IMPORTANT).
• Material considerations: Several bactericides are labeled with most applied pre-bloom, or at varying stages surrounding the bloom period, thus they should be noted for use in future spray programs. Materials include streptomycin containing compounds, fixed coppers, and P07 materials (fosetyl-Al, phosphorous acids and salts). Do not apply group P07 and copper-based fungicides within 14d of one another, as phytotoxicity is likely to occur!

Cankers diseases

• Be on the lookout for canker sporulation and oozing early this season. Common canker fungi include Botryosphaeria, Cytospora, Nectria, Phomopsis, Anthracnose pathogens, and Hypoxylon. It is critical to identity the causal pathogen in order to best detail material and timing considerations. Contact the Rutgers Plant Diagnostic Laboratory and/or your local agent to help with the diagnosis. In all cases, if possible, the canker should be removed during dry weather. Prune all diseased wood at least 6 to 8 inches below the infection, surface sterilizing tools between cuts (EXTREMELY IMPORTANT).

Boxwood Blight (BWB)

• If BWB is important to your business – Use the USPEST – PUSH Alerts – (click here) – to setup weekly email alerts. Create an account, watch the tutorial, and then set up your subscriptions. You select which locations (as many as you like), days you want to receive emails (pick Sunday/Monday), and what you would like to receive alerts about (BWB and a variety of other pests and diseases). Rutgers is not directly affiliated, however this is an incredible tool.
• The last updated Boxwood Blight Alert – use as a reference

Phytophthora, Pythium, Phytopythium root diseases 

• Compounds used to control Oomycetes (Phytophthora, Pythium, Phytopythium) are called Oomycides and are fundamentally different than those used for fungal and bacterial diseases. Therefor, proper identification is required, as a plant with a fungal root rot (black root rot or rhizoctonia) would require completely different materials. Contact the Rutgers Plant Diagnostic Lab for diagnostic services.  Within available materials there are three main groups, which describe where they will work on the plant given the application technique. Cultural practices are always more important than materials, especially in this case. 
  • Please review this handout for material and cultural practice considerations – Oomycete considerations in the nursery (Phytophthora, Pythium, Phytopythium, Downy mildews)
• Management focuses around treating the roots, or providing materials that reach the roots.
  • Protectants – are non-mobile, meaning they stay exactly where applied, must be root / drench / watered-in, applied.
  • Xylem mobile systemics – move upwards, and the roots must be treated.
  • Translaminar systemics – move very short distances into tissues, again meaning the roots must be targeted.
  • Amphimobile/Fully systemics (P07 / phosphonate group)– can move upwards to needles (xylem) and down to the roots (phloem) meaning foliage or roots can be targeted.

Refer to each label for treatment intervals as they vary greatly, however it is valuable to get ahead of this pathogen as very few materials can “cure” plants. Additionally, more than one material should be in your tool kit as Mefenoxam-resistance has been identified in the Mid-Atlantic region.

Do not apply group P07 and copper-based fungicides within 14d of one another, as phytotoxicity is likely to occur!

PLANT SPOTLIGHT

B. Errickson (Monmouth County RCE)

Our native NJ azalea species are currently in bloom, including dwarf azalea (R. atlanticum), pinxterbloom azalea, (R. periclymenoides), and swamp azalea (R. viscosum). These deciduous azaleas offer showy flowers that support pollinators and colorful fall foliage. As forest edge/understory plants, these species thrive in partial shade in well drained, acidic to slightly acidic soils.

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