Disease · Pistachio and mandarin
Alternaria: where the canopy is thinning
Alternaria alternata
Two different diseases share the name: one in pistachio and one in mandarins. Both make leaves drop. We fly and mark the zones with a thinner canopy, so you can check them on foot.
We have not flown this problem ourselves yet. We work it out with you, on your field.
The problem
Two diseases, one fungus group
In pistachio, dark spots a few millimetres across appear on the leaf, grow and merge until they reach 2 or 3 cm (about 1 in); on the nut, black dots. The first symptoms show in the upper branches in mid to late summer, and the leaf falls early. Moisture favours it: sprinkler irrigation that wets the canopy, dew, and a tall, dense cover crop.
In mandarin it is a different Alternaria, with a toxin that only attacks susceptible citrus. It dries shoots and young leaves and marks small fruit with dark, sunken lesions with a yellow halo. Some varieties, such as Fortune and Minneola, are more affected. It needs the leaf wet from dew or rain and around 25 °C (77 °F); mature leaves are not infected.
How it works
A flight to see the canopy
1
We fly the orchard
In pistachio, at the end of summer; in mandarin, after a wet spring or autumn, once leaves have already dropped.
2
We send you the map
Zones with a thinner canopy or dry shoots. The spots are millimetres across and cannot be seen from above: with a standard camera we only see the leaf that has already fallen, and other diseases look the same.
3
You confirm on foot
In pistachio, rub the spot: if it smudges black on your finger, that points to Alternaria rather than Botryosphaeria.
Where it shows up
Where it appears
- Pistachio
- Mandarin
What farmers ask us
Alternaria: field questions
Is it the same disease in pistachio and in mandarin?
No. The fungus belongs to the same group, but the one in mandarin carries a toxin that only attacks susceptible citrus.
Does the grass on the ground matter?
In pistachio, yes. A tall, dense cover crop holds moisture and keeps the leaf wet for longer.
What do I do in mandarin?
Remove and destroy the affected parts, open up the tree for air and avoid too much nitrogen, which gives tender shoots. Even so, preventive protection is usually needed: ask your adviser.
Know someone this would help?
Send them this page. If they have alternaria in their fields, you save them some searching.