basil diseases

If you love pesto and other recipes that feature sweet basil (Ocimum basilicum) you may already be looking forward to growing your own plants this summer!

Growing your own
Basil is an herb with tropical origins, so wait until after the last frost to sow seed or transplant starts into moist, well-drained soils in full sun. After the first few sets of leaves form, pinch back the tips to encourage bushy growth. Remove any flower buds for best flavor and to keep the plants producing more leaves. Harvest stems of leaves as needed; harvest entire top growth before frost.

Basil problems
Although insect pests are not usually much of a problem—aphids and Japanese beetles can do some damage, especially if your plants are just getting established—diseases are a concern!

• Basil Downy Mildew (BDM). Basil downy mildew (BDM) is caused by the fungus-like organism, Peronospora belbahrii that reproduces from spores spread by the wind and through seed saved from infected plants. BDM does not overwinter in Western North Carolina, but as the temperatures and humidity rise each spring, it is only a matter of time before winds bring it north.
  How to identify:
  – Leaf color changes—resembles nutritional problems that manifest as a yellowing or browning of leaves.
  – Dark spores on the underside of leaves—can be seen even without a microscope or hand lens.

  

  

• Fusarium wilt. Fusarium wilt (Fusarium oxysporum f. sp. Basilicum) is a soil and seed-borne disease that affects plants after they have grown normally to about 6 to 12 inches tall.
  How to identify:
  – Growth becomes stunted or wilts.
  – Stems show brown streaks and bend like shepherd’s crooks.

Preventing Basil Disease Problems
Prevention starts with healthy seed! Some seed companies test basil seed lots for fusarium wilt to reduce the risk of seed-borne illness.

Resistant varieties. There are also fusarium-resistant and BDM-resistant basil varieties. NC Extension Master Gardeners conducted trials in 2022 to compare five BDM resistant varieties (‘Rutgers Devotion,’‘Rutgers Obsession,’‘Rutgers Passion,’‘Rutgers Thunderstruck,’and ‘Prospera’) with the popular ‘Genovese’ variety.

In this project all of the resistant varieties remained disease free, while ‘Genovese’ was infested beginning in many locations in August (note that in 2019 Buncombe County reported BDM began in July)!

The resistant varieties compared favorably to ‘Genovese’ in taste tests and the top varieties were described as follows:

‘Prospera’—Growing taller and more upright than other varieties, Prospera was the overall favorite among the BDM-resistant cultivars. Some thought it needed staking because of its taller stature and a few volunteers reported stems breaking during storms.

‘Rutgers Passion’—This variety received positive comments on its color, aroma, plant vigor, and good branching habit.

‘Rutgers Obsession’—There was overall satisfaction with this cultivar based on its color and slightly more compact growth habit. It proved to be a solid BDM-resistant alternative to ‘Genovese’ and was the highest rated variety in taste tests.

Extension Master Gardener Volunteers Recommend These Varieties

Cultivation practices.

• Do not replant basil where soils may harbor Fusarium oxysporum f. sp. Basilicum, which may persist in soils for 8 to 12 years!
• Avoid overhead watering, which may contribute to disease spread.
• Fungicides that are available to home gardeners are not very useful in preventing disease.

Article by Barbara Hayes and Debbie Green, Extension Master Gardener^SM Volunteers.

For more information:

NCSU Plant Toolbox:  https://plants.ces.ncsu.edu/plants/ocimum-basilicum/

BDM resistant varieties
https://emgv.ces.ncsu.edu/2023/05/planting-basil-extension-master-gardener-volunteers-recommend-these-varieties/
New Sweet Basil Varieties from Rutgers University

Basil diseases:
NCSU Basil Downy Mildew Fact Sheet:  https://content.ces.ncsu.edu/basil-downy-mildew
https://www.buncombemastergardener.org/?s=basil+downy+mildew
https://growingsmallfarms.ces.ncsu.edu/growingsmallfarms-fusariumbasil/