Managing Alternaria Leaf Blight on Cantaloupe

Alternaria leaf blight (Alternaria cucurmerina) is frequently observed on cantaloupe crops in the Mid-Atlantic region. Symptoms of Alternaria leaf blight first appear as small tan circular spots with a white center on the upper leaf surface. As the disease progresses, the spots will enlarge and appear light brown.  Small veins within the spots will become dark in color, giving the spot a "netted" appearance. Eventually, the entire leaf may be covered with a series of brown circular spots.

Alternaria leaf blight can cause infected plants to defoliate heavily, resulting in sunburn injury to the developing fruits. A decrease in soluble solids and a loss in fruit quality may also be noticed by growers when Alternaria leaf blight is observed. Fruit infection may also be detected by Mid-Atlantic cantaloupe growers. Symptoms of fruit infection are characterized by circular, brown, and sunken spots. As the infection progresses a dark olive to black colored powdery mat will develop on the fruit surface. Infected fruit that is not culled at harvest or when packaged may decay in transit or storage.

Alternaria leaf blight can overwinter on crop debris or be introduced into a field by wind-borne fungal spores from infected crops or weed hosts. Warm temperatures and moist conditions will favor infection and disease development.

Alternaria leaf blight is best prevented by selecting resistant or tolerant varieties, practicing crop rotation, and utilizing protective fungicides to prevent infection. Growers wishing to protect their cantaloupe crop should initiate protective fungicide applications when the vines begin to run. Applications of Mancozeb 75DF (The varieties 'Harvest Queen,' 'Gold Star,' 'Super Star,' 'Sweet and Early,' and 'Saticoy' are sensitive to mancozeb) and Chlorothalonil 6F should be alternated with a tank mix of Chlorothalonil 6F and one of the following fungicides (with different FRAC codes) every 14 days. Fungicides recommended in the 2022-2023 Mid-Atlantic Commercial Vegetable Production Recommendations for managing Alternaria leaf blight include:

• Pristine 38 WG (FRAC code 7 + 11) *Tank mixes of additives, adjuvants, and/or other products may result in crop injury.
• Inspire Super 2.82EW (FRAC code 3 + 9)
• Quadris Top 1.67SC (FRAC code 3 + 11) *Do not apply near apple trees.
• Luna Sensation 4.25SC4 (FRAC code 7 + 11) *A mild yellowing on leaf margins is sometimes seen following the application of Luna Sensation in cucurbits.
• Aprovia Top 1.62EC (FRAC code 3 + 11)
• Merivon 2.09SC (FRAC code 7 + 11) *Tank mixes of additives, adjuvants, and/or other products may result in crop injury.
• Topguard EQ 4.29SC (FRAC code 3 + 11) *Do not apply near apple trees. **Do not tank mix with crop oil concentrates, methylated spray oil, or silicon adjuvants. Do not tank mix with Malathion, Thiodan, Lannate, MPede, or Botran.
• azoxystrobin 2.08F (FRAC code 11) *Do not apply near apple trees. **Do not tank mix with crop oil concentrates, methylated spray oil, or silicon adjuvants. Do not tank mix with Malathion, Thiodan, Lannate, MPede, or Botran.
• Cabrio 20EG (FRAC code 11) *Tank mixes of additives, adjuvants, and/or other products may result in crop injury.
• Reason 500SC (FRAC code 11)

Reference/Source

Bernhardt, Elizabeth, et al. Cucurbit Diseases: A Practical Guide for Seedsmen, Growers, and Agricultural Advisors. Petoseed Company, 1988.