Phytophthora Gummosis of Citrus in Northern and Southern Iran

Amber gum on the collar and the lower trunk, a sudden yellowing of one side of the crown, and the gradual death of a citrus tree in the orchards of Mazandaran, Gilan, Jahrom, Darab and Hormozgan are the familiar picture of Phytophthora gummosis. The agent lives in soil and water and enters through a collar wound, a deeply buried graft union, or water splashed onto the trunk. The disease is seen in two completely different climates of the country: the humid north with rain and heavy soil, and the south with flood or irregular drip irrigation on soil that keeps the collar wet. Chemical treatment plays a secondary role when the collar is permanently wet.

Gum, the collar, and what a grower must see the same day

Phytophthora kills living bark. The tree exudes gum to limit the wound. If you carefully lift the collar bark, you see brown to chocolate tissue beneath it with a clear boundary against healthy tissue. Sometimes the lesion runs right round the trunk and the tree collapses within a season; sometimes it takes only one side, and the crown on that side weakens. The common mistake is to scrape widely and leave the wound exposed to sun and rain unprotected. The scraping should be limited to dead tissue, and the surface must not be made so large that healthy vascular tissue is destroyed. Every tool that touches a wound must be disinfected between trees; otherwise the grower is the one moving the pathogen around.

Gummosis also comes with root rot. In that case the tree may show little gum, but growth stops, the leaves are small and the fruit drops before ripening. Digging at the collar and finding black, rotten roots completes the diagnosis. In the north this pattern is more common after rainy winters and on plots without drainage. In the south it is seen after heavy irrigation cycles or when a dripper at the foot of the trunk clogs. A late diagnosis is usually because the grower looks at the crown first and does not inspect the collar until the tree has yellowed on one side.

Drainage and planting depth: two decisions that bring the disease forward

The graft union must be above the finished soil level. Deep planting — sometimes done to establish the sapling better — keeps the bark of the rootstock and scion in permanent contact with moist soil. If the graft is low in the nursery, that sapling is condemned in the orchard. In a bearing orchard, soil and mulch must not pile up on the collar. Irrigation must keep the wetting bulb in the root zone rather than wetting the trunk. In a drip system, keep the line away from the trunk. Under flood irrigation, making a basin around the tree with no outlet drowns the collar.

• On northern plots with high clay, subsurface drainage or at least an outlet channel is essential.
• After prolonged rain, inspect the collars and push aside any accumulated soil.
• Plant saplings on a ridge so the collar stays above the seasonal water table.
• Do not let air-conditioner water, sewage or standing street water into a citrus orchard.

Copper and a resistant rootstock are tools, not miracles

Copper compounds are used on the scraped wound and preventively on the collar during wet periods. Copper does not drive the disease out of an orchard whose collars are standing in water. Repeating copper without restraint both burns the bark and contributes to metal accumulating in the soil. A rootstock resistant to Phytophthora — depending on the region and the citrus viruses prevalent there — reduces the risk of the pathogen entering from the soil, but it does not insure the tree against deep planting and flooding. The choice of rootstock has to fit the climate of the north or the south, the salinity of the water and the viral diseases of that district, not one resistance trait alone.

1. Monthly collar inspection in the wet season; manage fresh gum in the same week.
2. Limited scraping of dead tissue, disinfected tools, protection of the wound surface.
3. Correct the slope, the ridge, the dripper distance and the outlet for standing water.
4. Copper at a defined time and dose, not as a trunk drench at every round.
5. Gradual replacement of trees on a susceptible rootstock on plots with a history of gummosis.

The humid north and the hot south: one pathogen, two water routes

In Mazandaran and eastern Gilan, autumn and winter rain saturates the clay soil and the collar stays wet for weeks. Here the priority is drainage, ridging and keeping mulch away from the trunk. In Jahrom, Darab, Minab and Hajiabad there is less rain, but flood irrigation or a dripper against the trunk creates exactly the same wet environment. A southern grower sometimes assumes that a dry climate keeps gummosis away; the fact is that the pathogen needs local moisture at the collar, not a humid climate. After every heavy irrigation, look at the base of the tree: if the collar is muddy or still glistening, increase the distance of the dripper and open the standing basin. Plant new saplings on a ridge even if the old orchard is flat; one corrected row over five years works better than replacing gum-affected trees too late. Do not attribute fruit drop and small leaves to nutrition alone until you have looked at the collar. On plots with a history of gummosis, do not buy a sapling on a susceptible rootstock even at a low price.

Phytophthora gummosis in the citrus of northern and southern Iran is a soil-borne disease with the face of irrigation. The gum is the symptom; the cause is usually a wet collar, deep planting and poor drainage. Copper and a resistant rootstock mean something only once those causes are removed. The orchard's programme has to start from the planting design and the water route, not from the pesticide shelf. When advising citrus orchards, Farin Kesht Taban looks at the collar and the drainage first, and only then moves to treatment.